Top Jamaican government officials said tonight that Caribbean leaders made a formal request for U.S. military intervention in Grenada only after Washington signaled its willingness to invade the island.

The officials said the Reagan administration has been seeking for several months to have the Caribbean Common Market nations "isolate" Grenada as a "communist outpost" in the region. They also said that the United States had urged the Caribbean nations to consider military action against Grenada.

Jamaican Prime Minister Edward Seaga, the island's first leader in several years to be supportive of the United States, told Parliament yesterday that after a meeting of Caribbean officials in Barbados last Friday it was agreed to see if the United States would participate in an invasion. After the Reagan administration promised "immediate action," one official said the Caribbean leaders then wrote a formal request for U.S. military help on Saturday.

However, Vaughan Lewis, director general of the Organization of Eastern Caribbean States, headquarted in Barbados, said the decision to seek American troops was made only after the member nations found they did not have sufficient manpower to take control of the island.

Trinidad and Tobago and Guyana reportedly said they would not commit men to the invasion force. That left the Caribbean leaders with a choice of either backing off their invasion plans or seeking U.S. or British help. The British declined to help, but the United States promised immediately to commit troops, Lewis told Washington Post correspondent John Burgess in Bridgetown.

"It was a decision that was very, very difficult," said Lewis. "It was a decision in the last resort that was the only alternative."

Lewis said the Caribbean nations were aware that they could be accused of being "American lackeys. It's a problem that we had."

Jamaican officials said today that a 50-man police force from St. Kitts, St. Vincents and Barbados, has been organized to enter Grenada and restructure that country's police force.

Prime Minister Seaga has argued here that the Caribbean community's action in Grenada is proof of a "turning point in the history of the English-speaking Caribbean." Seaga justified the invasion by saying that Grenada was becoming a threat to the rest of the Caribbean countries.

"In the states in the eastern Caribbean, there is at this moment not only a strong revulsion against the recent atrocities in Grenada," Seaga said in a statement to the nation released today, " . . . but also overwhelming anxiety--indeed fear for their own security. This added urgency to the need to find a solution to the Grenada crisis.

"The time has now come when the English-speaking Caribbean countries have made it unmistakeably clear that they will not tolerate subversion and revolution . . . . Today's Caribbean leadership is determined that instances of military and revolutionary takeovers must be dealt with in such a manner as will leave no room for doubt whatsoever as to the will of the majority of the English-speaking Caribbean."

Reaction to the invasion here was marked more by fascination than by opposition to Seaga, who played the leading role among the Caribbean leaders. There was no talk of demonstration and opposition leader Michael Manley, the former prime minister, said in a speech to the Parliament that while he was not in favor of the invasion, he felt it was done with "good intentions." Manley has since left the country, keeping scheduled appointments in New York.

P.J. Patterson, chairman of the opposition People's National Party, and Manley's foreign minister, said opposition is based on the violation of the principle of sovereignty.

"We indicated through party leaders that we are opposed to intervention in principle," Patterson said in an interview. The party "had made clear our repugnance for the military regime in Grenada and said everything ought to be done to isolate them. But intervention opens the door to many dangers by legitimizing a country or group of countries stepping into a sovereign nation in the name of restoring order."

Seaga answered similar questions in heated parliamentary debate yesterday by raising the threat of further takeovers of Caribbean nations.

"It may be felt that these matters do not concern us," Seaga said of the assassination of former Grenadan prime minister Maurice Bishop. "But most certainly they do. If a whole government can be wiped out overnight either by political or military extremists and the governments of the Caribbean remain silent and passive, then no government elected by the people can be safe from madmen of one type or another who would seek to replace a government of the people."

"Obviously when you get involved in a war everybody has to ask themselves if it was right to do it," said Roy Collister, president of the Private Sector Organization of Jamaica. "But in this case it is not as if we attacked some good boys. What happened was that country was a threat to the entire Caribbean."

Conversations with unofficial Jamaicans and with diplomats here confirmed that widespread opposition--in the form of demonstrations or calls for resignations of leaders--is unlikely because the assassination of Bishop terrified Caribbean residents.

One story given prominence on radio and in newspapers here charged that a busload of children was killed by Bishop's assassins. Seaga in his speech announcing the invasion mentioned a report that one child's legs were "blown off" as the child tried to escape from the assassins.

The loudest regional voices of opposition were of George Chambers, prime minister of Trinidad and Tobago, who in radio broadcasts said he found the invasion "regrettable." Guyana's President Forbes Burnham similarly said that he opposed the invasion because it was an attempt to "influence the political direction of a people."