The last American peace-keeping troops left Grenada today, formally ending a 20-month occupation that began by destroying this island's Marxist government and led to a new security system of U.S.-trained military forces in the eastern Caribbean. Thirty Special Forces GIs are staying here until September to do that training.

The departing U.S. contingent, 66 soldiers with jeeps, equipment and a mongrel dog, took off in a C141 bound for Maguire Air Force Base, N.J., and a C130 bound for Ft. Bragg, N.C., officers said. Prime Minister Herbert Blaize, a frail 67-year-old notary public elected last December, presided from the back seat of his official sedan over a rainy departure ceremony at the Cuban-built Point Salines Airport.

Blaize, who had asked the Reagan administration to keep U.S. forces on the island, said in an interview that he has become reconciled to the departure despite some apprehension among Grenada's 90,000 residents. He said many islanders have remained scarred by a bloody coup against then-prime minister Maurice Bishop in October 1983 and the subsequent U.S. invasion that restored democracy here after four years of Marxist rule.

"A lot of people are still emotionally concerned," he said. "The Americans are still a symbol of security, and if that symbol is removed, some people naturally are afraid."

The 30-man U.S. Special Forces team staying until September is training members of the eastern Caribbean's new Special Services Units, uniformed security forces with U.S. military training and U.S.-supplied M16 automatic rifles, machine guns and grenade launchers. Two 40-man platoons are being trained at Camp Salines here for each of the island nations of Grenada, St. Vincent, St. Lucia, Dominica and St. Kitts.

Blaize said a treaty also has been prepared to join these islands along with Barbados and Antigua, which already have paramilitary security forces, in a U.S.-oriented mutual defense pact allowing the leader of one island to call for military intervention by the Special Services Unit of another.

Barbados and the Organization of Eastern Caribbean States have been joined in a loose Regional Security System since 1981, when they joined forces against the leftist Bishop government then ruling Grenada along Marxist lines. But U.S. introduction of the Special Services Units and the formal mutual defense treaty under way mark a new departure in military training, equipment and coordination in the traditionally peaceful eastern Caribbean.

One of the neighboring islands' main complaints against Bishop between 1979 and 1983 was his attempt to build a 2,000-man army with advisers and equipment from Cuba and other communist countries.

U.S. diplomats said island leaders have discarded an earlier idea for a standing 800-man security force, pushed by the late prime minister Tom Adams of Barbados. Cost was the main reason for the shift to smaller Special Services Units, they added.

Some Caribbean leaders have expressed fears the Special Services Units, although small, could on the Caribbean scale evolve into centers of political power -- like Latin American armies.

Police Commissioner Russell G. Toppin said this concern has been discussed and, partly as a result, each island will maintain only one of its two platoons on active duty, filtering the other into regular police duty.

Blaize, asked about these concerns, said, "You can't have your cake and eat it, too."

U.S. forces, which numbered more than 6,000 at the height of the Oct. 25, l983, invasion, have stood at about 250 for the last year. But they have been gradually reducing their presence here since April, leaving mostly support troops.

Their half-dozen helicopters departed last week. At a ceremony yesterday, the U.S. flag was lowered from the 184-room Grenada Beach Hotel that has been their beachside barracks for nearly two years. The government has announced a Trinidadian businessman plans to reopen it to tourists.

British experts have helped train a 550-man Grenadian police force to provide everyday security. Toppin, a 36-year veteran of the Trinidad force hired here on a three-year contract, said he plans to acquire automatic weapons for Grenadian policemen to increase their dissuasive power.

In addition, a handful of Jamaicans, Barbadians or Antiguans, remnants of the U.S.-sponsored Caribbean Peace Force, are likely to remain on hand for a number of months to supervise Grenadian guards at the Richmond Hill Prison, where 19 former officials in the government of Bishop still await trial for his killing, Toppin said.

Blaize said he remains convinced that some of Bishop's followers in the New Jewel Movement, reorganized as the Maurice Bishop Patriotic Movement, have arms hidden about the island and that "those wretches" are "always looking for the opportunity to make trouble."