The Reagan administration justified its coordinated attack on Grenada yesterday as a preemptive strike to prevent about 1,100 Americans living there from being taken hostage and as a response to urgent requests from six Caribbean countries to restore order and stop the spread of Marxist revolution in the region.

President Reagan, Secretary of State George P. Shultz and senior administration officials said an "atmosphere of violent uncertainty" in the small island nation prompted a series of high-level meetings late last week and through the weekend, leading to a signed presidential invasion order at 6 p.m. Monday. That decision was backed, Reagan said, by Jamaica, Barbados and an alliance of other eastern Caribbean countries.

The concern of these nations, after the bloody coup last week in Grenada that pitted Marxists against Marxists, was summed up in a statement by Prime Minister Eugenia Charles of Dominica, who appeared at Reagan's side as he announced the invasion yesterday morning.

"It is not a matter of an invasion," she said. "It is a matter of preventing this thing Marxist revolution from spreading to all the islands."

"The United States' objectives," Reagan said, "are clear: to protect our own citizens . . . and to help in the restoration of democratic institutions in Grenada," a former British colony that has been under the control of a Marxist regime since 1979.

The leading figure of that regime was Maurice Bishop, 39, who took power in a bloodless coup in 1979 and steered the island of 110,000 people into a close relationship with Cuba.

Bishop was placed under house arrest two weeks ago and then killed after a crowd of supporters who freed him last Wednesday was fired on by the island's armed forces. Most of the members of Bishop's cabinet also were reported killed.

Last night, as fighting continued, the Pentagon said at least two American soldiers had died and 23 were wounded. In addition, the Pentagon said, more than 200 armed Cubans were taken prisoner, and the Cuban news agency Prensa Latina said the remaining Cubans, numbering about 400, had been ordered not to surrender.

Reagan last night also formally notified House and Senate leaders of the invasion under the War Powers Resolution, but he did not invoke a section of the law that might require withdrawal of American troops if Congress did not approve.

U.S. officials also reported an undetermined number of casualties among Cubans on the island who have been assisting in the construction of a new airport that the Reagan administration feared would become a resupply base for Cuba and the Soviet Union.

Shultz said U.S. assault troops had exchanged fire with the Cubans, who later in the day returned to their barracks, according to one official, and would soon be escorted to a Cuban ship.

About 30 Soviet advisers on the island were reported safe and being treated with "diplomatic courtesy," and U.S. officials said they were making provisions for them to leave Grenada.

The American civilians on Grenada were safe last night, officials said. About half of them, students at a medical school in St. George's, the capital, were said by administration officials to be in a high state of "anxiety" before the invasion because they were not free to leave or move about the island, which had been under a "shoot-to-kill" curfew order.

Since Bishop's death last week, the island has been under the control of the chief of Grenada's armed forces, Army Gen. Hudson Austin. A Senate source briefed by administration officials yesterday said U.S. intelligence agencies had information that a "counter-coup" was imminent and that Austin's control was in doubt before the invasion.

Shultz said the final invasion order was signed by the president Monday evening after consultation with the Joint Chiefs of Staff. The airborne assault on the island began 11 hours later. As it progressed, the invading U.S. Marines and Army Rangers, of whom as many as 1,900 were on the island at a time, were joined by 300 soldiers from Jamaica, Barbados, Dominica, St. Lucia, Antigua and St. Vincent.

Reagan, Shultz and other senior officials coupled what they described as a potential threat to the lives of U.S. citizens with the judgment of the Caribbean governments working with the United States that the emerging Marxist leadership of Grenada would threaten its neighbors.

"We see no responsible government in the country," Shultz told reporters. "We see arrests of leading figures" and "executions."

"All of these things are part of an atmosphere of violent uncertainty that . . . caused anxiety among U.S. citizens and caused the president to be very concerned about their safety and welfare," he said. According to Shultz, Reagan "felt that it was better under the circumstances to act before they Americans might be hurt or taken hostage than to take any chance given the great uncertainty clearly present in the situation."

Shultz said the military action did not violate the charter of the Organization of American States, which forbids intervention by members in each others' internal affairs. Shultz said a mutual security treaty among eastern Caribbean countries, including Grenada, justified the United States' working in concert with Grenada's neighbors to eliminate the threat of a potentially more hostile government there.

A senior White House official reiterated this point by saying Grenada itself had acknowledged in recent weeks that it effectively had no government, thus providing a legal basis for triggering the collective-security provision of a 1981 treaty of the Organization of Eastern Caribbean States (OECS).

The dramatic military action, which took members of Congress by surprise, appeared to raise significant diplomatic and political questions about the use of U.S. military power in the Caribbean Basin and set off a new debate over the president's authority to commit U.S. troops under the War Powers Resolution.

"I would like to shout from the rooftops that the president must explain what legal grounds exist for the actions he took," said Sen. Daniel Patrick Moynihan (D-N.Y.), acting chairman of the Senate Intelligence Committee.

In addition, British Prime Minister Margaret Thatcher announced in London that she had counseled the White House against the invasion. Shultz answered by saying, "We responded to the urgent request of the states in the area . . . they're no longer British colonies . . . . The Caribbean is in our neighborhood."

Shultz also revealed that U.S. officials received strong diplomatic protests from the Soviet Union and from Cuba after the State Department notified them that the invasion was under way.

Reagan canceled a political speaking tour in Dallas and Las Vegas later this week to deal with the crisis and the White House announced that the president will give a nationally televised address Thursday evening.

Officials said the administration plans to remove American troops from Grenada as soon as possible and allow its neighbors to help set up a provisional government leading to democratic elections there. According to Sen. Charles H. Percy (R-Ill.), chairman of the Senate Foreign Relations Committee, after a White House briefing yesterday morning Gen. John W. Vessey Jr., chairman of the Joint Chiefs, said the troops will be out "in a week, if it's possible."

Rumors about a possible invasion had washed through Washington Monday night. From Bridgetown, Barbados, reporters observed U.S. military planes and helicopters ferrying U.S. troops through the area. House and Senate leaders were called to the White House or tracked down at speaking engagements to be briefed on the imminent invasion.

Only hours after early-morning radio reports from Grenada reported fierce fighting on the island, President Reagan appeared in the White House briefing room and said: "Early this morning, forces from six Caribbean democracies and the United States began a landing or landings on the island of Grenada . . . ."

Reagan said he had authorized the invasion to "protect innocent lives" and "forestall further chaos" there and to restore "democratic institutions" in the wake of a bloody coup in which "a brutal group of leftist thugs violently seized power."

Declaring that "American lives are at stake," Reagan said he "received reports that a large number of our citizens were seeking to escape the island, thereby exposing themselves to great danger" and "I concluded the United States had no choice but to act strongly and decisively."

Administration officials said yesterday that they had received no reports of direct threats against the Americans on the island. But Reagan said "there was no way" Americans could get off the island and "this was a case of not waiting until something actually happened to them."

The events that led to the invasion began gathering steam in Washington last week.

Thursday afternoon, after word of Bishop's execution the day before reached the capital, Reagan asked Vice President Bush to chair a meeting on Grenada at the White House.

Shultz said he got to the meeting late, after a closed-door session on Capitol Hill. He arrived with Assistant Secretary of State for Inter-American affairs Langhorne A. Motley. Shultz said the session was devoted to reviewing "the grave turn of events" in Grenada and to consider the impact on Americans there.

The group decided to recommend to Reagan that he divert a large naval force to the area. Among them were ships carrying Marine replacements for the multinational peace-keeping force in Lebanon, including those killed in Sunday's bomb attack in Beirut. Shultz said the diversion was "essentially precautionary."

Reagan agreed to divert the ships. The next day, Friday, the president left for what was supposed to be a leisurely golfing vacation in Augusta, Ga., with Shultz, Treasury Secretary Donald T. Regan, former New Jersey senator Nicholas Brady and their wives. There were private discussions about the Grenada situation on Friday, Shultz said.

In a different set of meetings in Bridgetown, Barbados, that day, the eastern Caribbean leaders, fearful over what was happening in Grenada, made an "informal" appeal to the Reagan administration for help, according to White House spokesman Larry Speakes.

Shultz said a message from Barbados reached him in Augusta at 2:45 a.m. Saturday. The diplomatic cable, in which the OECS states were joined by Jamaica and Barbados, stated "their very strong feeling" that they could not respond on their own to the Grenada situation, Shultz said.

Shultz shortly thereafter discussed the cable with the new national security affairs adviser, Robert C. McFarlane, who was in Washington. Bush had convened a National Security Council meeting in Washington and Shultz joined in by conference call "to evaluate the situation and the information in the cable."

"We shortly got the president up," Shultz said, and brought him up to date. Reagan spoke by phone with Bush and Secretary of Defense Caspar W. Weinberger, and "gave his own reactions," Shultz said.

At 9 a.m. Saturday, Bush started another meeting in Washington and Reagan spoke with him again. Reagan and Shultz talked about returning to Washington but decided against it, Shultz said, because they feared a sudden change in plans would call attention to the crisis, which was then a secret known only to top administration officials.

But the Beirut bombing early Sunday changed their plans, and Reagan rushed back to the White House. Later, the White House received a formal plea from the eastern Caribbean nations to intervene in Grenada.

Early Sunday, former ambassador to Costa Rica Francis J. McNeil and Maj. Gen. George B. Crist of the Joints Chiefs of Staff were sent to Bridgetown to hold talks with the Caribbean leaders. Throughout the afternoon and evening, they were in telephone contact with the White House, Shultz said.

Reagan tentatively decided late Sunday to lead the OECS effort and to commit American military forces. At that time, there appeared to be a "very uncertain and violent situation" threatening American citizens on the island, Shultz said.

Also on Sunday, the international airport at Grenada was opened for two hours so American diplomats could arrive to speak with U.S. citizens, according to the White House. Despite the report of a top official at the medical school on the island that there was no immediate threat to the students, the diplomats reported they found them in "high anxiety" because of restrictions on their movements and the shoot-to-kill curfew, a senior official said yesterday.

On Monday, Reagan began reviewing the military plans for the invasion with the Joint Chiefs of Staff and Weinberger at a meeting from 2:15 to 3:30 p.m., Shultz said. Reagan made a "semi-final" military decision at the end of that meeting. At about 6 p.m., he signed the directive ordering the invasion. McNeil returned from Barbados that day.

Key congressional leaders were immediately contacted and brought to the White House for an 8 p.m. session with Reagan. They included House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass), Senate Majority Leader Howard H. Baker Jr. (R-Tenn.), House Minority Leader Robert H. Michel (R-Ill.), House Majority Leader James C. Wright Jr. (D-Tex.) and Senate Minority Leader Robert C. Byrd (D-W.Va.).

Reagan was joined by Shultz, Weinberger, Vessey, presidential counselor Edwin Meese III, chief of staff James A. Baker III, deputy chief of staff Michael K. Deaver, presidential assistant Richard G. Darman and congressional liaison Kenneth M. Duberstein.

The congressional leaders were told of Reagan's plans but were not given a timetable.

After the invasion began about 5:30 a.m. yesterday, Reagan awoke about 6:30 a.m. and began making telephone calls to his staff and others, Speakes said.

At 7:30 a.m., Reagan met in the Oval Office with Prime Minister Charles, Shultz, Weinberger, McNeil, Motley, Meese, Deaver and a Latin-American affairs expert from the National Security Council. This group left 28 minutes later, with Shultz and Weinberger remaining to brief Reagan on developments.

At 8:14 a.m., Reagan went to the Cabinet Room to brief a larger bipartisan congressional leadership group, giving a 10-minute statement underscoring the reasons for the invasion before turning to McFarlane and Weinberger for details.

At 9:07 a.m., Reagan went on national television to make the announcement. Then he returned to the Oval Office and opened a Cabinet meeting on the crisis.