President Reagan, shifting his justification for the American invasion of Grenada, said last night that U.S. forces had discovered a "complete Cuban base with weapons and communications equipment" that made it "clear a Cuban occupation of the island had been planned."

Saying that U.S. troops found warehouses with "weapons and ammunition stacked almost to the ceiling, enough to supply thousands of terrorists," the president called Grenada "a Soviet-Cuban colony being readied as a major military bastion to export terror and undermine democracy."

While the administration presented only fragmentary evidence of what Reagan called the major Cuban buildup, the president said in his nationally televisied address, "We got there just in time."

Hours earlier a senior Pentagon official, also referring to what he called signs of a Cuban buildup on the island, said U.S. forces may have to remain there "indefinitely" to combat likely Cuban efforts to return, inspire guerrilla activity and regain control.

"I don't think the Cubans are going to give up," said the official, who declined to be identified.

He observed that it would take a large naval and ground force to keep Cuba from sending small boats with weapons to Grenada in ensuing months.

Initial response in Havana early today came from deputy foreign minister Ricardo Alarcon, who told Washington Post correspondent Alma Guillermoprieto that Reagan's description of Cuban military installations in Grenada was "a great lie . . . because we don't have any military installations or structures in Grenada."

The administration had said earlier--and still was saying officially yesterday--that the U.S. military operation on Grenada would be surgical and brief.

The Pentagon said yesterday that eight U.S. servicemen had been killed in the invasion, eight others were missing and 39 were wounded.

The theme of Cuban involvement on the island built in a crescendo yesterday, with administration officials repeatedly raising their estimates of the Cuban presence, saying they had been surprised by it and citing it as justification for the action.

There had been almost no mention of Cuba at the outset. Officials had said then that the invasion was ordered to protect U.S. citizens on the island and to support neighboring island governments that had asked for help.

Most of the Americans on Grenada were students and faculty members at St. George's University School of Medicine, 409 of whom had been flown to the United States by yesterday, officials said.

Administration officials said U.S. forces had encountered twice as many Cubans in Grenada as expected and discovered a "combat engineer battalion" and a Cuban colonel on the island. One official said the "indications are they were building a major Cuban base."

Late in the day, Pentagon officials showed reporters videotapes that were said to have been made early yesterday morning in Grenada and that showed six warehouses of Soviet-made small arms ammunition and weapons. An unidentified U.S. colonel narrating the tape said the weapons cache, discovered five miles north of Pt. Salines airport, was "far above what any force on any island this size would need for self-defense."

The weapons shown in the videotape were small arms, assault rifles, machine guns, mortars and, in one case, an antiaircraft battery that the unseen narrator described as "extremely lethal." It was impossible to gauge the amounts of weapons discovered, however, and the tape did not reveal any larger equipment that might belong to an aggressive or invading force such as helicopters, ships, tanks, missiles or armored personnel carriers.

After a White House meeting with Reagan yesterday, Prime Minister John Compton of St. Lucia, one of the eastern Caribbean nations that participated in the invasion, said the Cuban "military buildup" was the main reason that his nation and others had sought U.S. military help.

"The United States came to our aid because we thought the military buildup was threatening the whole of the southern Caribbean," he said.

In the one clear response to the invasion on Capitol Hill yesterday, the House Foreign Affairs Committee adopted, 32 to 2, a declaration that the War Powers Act of 1973 applied to the use of American troops in Grenada. As approved, the resolution would require the removal of the troops after 60 days with the clock starting Tuesday, when the invasion began.

Also yesterday, Federal Communications Commission officials said they were reiterating standing rules when they sent an advisory Wednesday to amateur or "ham" radio operators and commercial broadcasters that amateur frequencies could not be used to transmit news stories. The advisory was sent when administration officials still were refusing reporters permission to cover the invasion.

After sharp criticism from news organizations across the nation, the administration eased a news blackout it had imposed on coverage of the first two days of the invasion. The Pentagon late yesterday gave 12 reporters an escorted glimpse of Grenada.

On the Cubans, Reagan's new national security affairs adviser, Robert C. McFarlane, told reporters yesterday, "The discoveries that have been made are extraordinary in terms of capability and infrastructure and what it portended for the future."

Reagan last night linked the Soviet Union to the Cuban presence. He said it was "no coincidence" that when a leftist military junta took over earlier this month, "there were 30 Soviet advisers and hundreds of Cuban military and paramilitary forces on the island." Administration officials had said previously that the 20 to 30 Soviets on the island were embassy officials.

A senior administration official, briefing reporters at the White House before the president's speech, said that there were about 1,100 Cubans on the island, more than twice as many as previously believed, and that several hundred remained at large.

It had been known that there were Cuban construction workers on Grenada helping to build a large airstrip that the administration had feared could be of possible military use to hostile powers. But the construction workers had not been considered a military force.

The official said the weapons stores on the island were "sufficient to equip terrorists in the thousands." He said this meant that the Cubans apparently intended to "exploit Grenada as a forward Cuban base." He added that the military preparations on Grenada were "enough to sustain a significant ground force or terrorist operations."

This official added that U.S. forces had discovered large quantities of automatic weapons, mortars, ammunition and equipment for light infantry during the invasion. He added that "extremely sophisticated" communications equipment had also been discovered.

Presidential spokesman Larry Speakes told reporters earlier yesterday, "Clearly you had a substantial Cuban organization there that was well equipped. It was following a pattern of previous Cuban behavior in Angola and Ethiopia."

The strong emphasis on Cuba's military buildup contrasted with the administration's insistence in the first two days of the invasion that the action was motivated by concerns about the safety of American citizens in Grenada and was designed only to restore order and "democratic institutions" on the island.

The eight-minute videotape which the Pentagon unveiled for reporters tonight was filmed at 5 a.m. yesterday, Pentagon officials said. It showed six moderate-sized warehouses roofed in tin or plastic sheeting, and at least two sheds covering what a narrator said were Soviet made heavy trucks and ambulances, all surrounded by a low barbed-wire fence.

Inside the warehouses were hundreds of crates of ammunition for machine guns, Soviet made AK47 automatic rifles, 120mm mortars and anti-aircraft guns, the narrator said. Some of the crates were marked with Russian letters and others had stamped on them, "Oficina Economica Cubana."

The narrator said U.S. forces found "millions of rounds and thousands of weapons" at the complex which appeared to be surrounded by low green vegetation not far from the ocean. The film also showed several hundred men who were said to be Cuban prisoners being guarded by Barbadan and possibly other Caribbean nations' soldiers.

The prisoners were lying close together on the pavements, many of them covering themselves with large straw hats from what was apparently hot sun. Some of them appeared to be in their early 20s in khaki uniform, while others were quite bedraggled and apparently middle-aged.

Even as the administration was disclosing what it described as surprising evidence of Cuban military involvement in Grenada, officials suggested the possibility of a longer stay for U.S. forces than was originally forecast.

Administration officials repeatedly have refused to set a specific timetable for withdrawal. Last night, Reagan said, "It is our intention to get our men out as soon as possible." A senior administration official said it was expected U.S. troops might remain some weeks, but not months.

A senior Pentagon official said he believed that the United States will try to install a force from neighboring Caribbean nations, Great Britain or elsewhere that might be able to take the place of U.S. forces and still maintain order. This official, however, was pessimistic on the chances of finding a force capable and willing to do so.

"The tendency in government is to worry about what has to be done now, and then worry about the rest later," he said, suggesting that the Pentagon may not have had time to plan carefully for the occupation during the few days that the invasion was put together.

"Over the next six months, I just hope to God we can find a force that can maintain order and prevent infiltration," he said. "I would think neither Defense Secretary Caspar Weinberger nor Gen. John Vessey chairman of the Joint Chiefs of Staff wants to pin us down any further than we're pinned down already, but you can't leave a vacuum . . . . They could just establish a constabulary, if it weren't for the Cubans."

Administration officials originally had envisioned a police force for Grenada made up of troops from the Caribbean nations that joined the invasion. But Compton, the St. Lucia prime minister, suggested that the island nations had at their disposal a combined force of only about 500 men. He said leaders of the Organization of Eastern Caribbean Nations, joined by leaders of Jamaica and Barbados, would meet Saturday to discuss the post-invasion force.

Compton also predicted that U.S. forces would be needed in Grenada for only a matter of weeks.

Administration officials said that the island's governor-general, Sir Paul Scoon, was returned to Grenada yesterday from the USS Guam and was to give a radio speech to the populace. Scoon had been held under house arrest after the 16-member military junta seized control of the island.

Compton said yesterday that before the invasion Scoon had written a letter seeking help for the island from the other Caribbean states. But the letter, dated Monday, Oct. 24, the day before the invasion, wasn't delivered until after Scoon was removed from his house by U.S. forces in an armored personnel carrier.

Compton said word of the request had been passed to the OECS states by an unidentified western ambassador on Grenada.

He added that the Cuban buildup on the island had been under way since 1979, but while former prime minister Maurice Bishop was in control he had a "restraining hand" on the leftist forces on Grenada. Bishop was placed under house arrest two weeks ago by the island's armed forces and was killed Oct. 19.

Compton said that the Caribbean nations expect a provisional government to hold elections in about six months. The administration has provided no timetable.

In London, Commonwealth Secretary General Shridath Ramphal urged that the group create a peace-keeping force for Grenada to replace the Marines, which he said should be withdrawn from the island "very, very quickly."

One question still unresolved is how the administration intends to evacuate at least 600 Cuban prisoners on the island whom Reagan said "will be sent to their homelands."

There were lingering questions at the White House yesterday about how the administration had tried to control news coverage of the invasion. Speakes, responding to The Post's report that he had discussed resigning after senior officials misled him about the invasion, said, "the thought never crossed my mind."

Speakes told reporters that he had consented to allow the Pentagon to manage the flow of news about the military actions because "that is the way they want to do business and I have no objection to that." The Pentagon continued to restrict tightly the amount of information available about the invasion, reportedly on the orders of Gen. Vessey.