Bringing in U.S. Green Berets to train a commando squad here undermines the country's tradition as an unarmed, neutral democracy, the national leader who abolished Costa Rica's Army has said.

"We don't want a group of jackbooted young men here, goose-stepping, saluting and clicking their heels," former president Jose Figueres said in an interview with the English-language weekly Tico Times published yesterday. "This would mean the beginning of militarism in our country. I see in this another U.S. mistake in Latin America in the making."

Figueres, a 78-year-old elder statesman who disbanded the Army after winning a 1948 civil war, was commenting on the recent arrival of about 20 Green Berets to begin training 750 antiterrorism commandos in the Costa Rican Civil Guard, the first such unit in the history of the 7,000-man paramilitary force.

His words attracted special attention because Figueres has been president three times and still helps lead the ruling National Liberation Party. In addition, his gesture more than 36 years ago is considered a key reason Costa Rica has been the only Central American country to maintain a strong democracy without military domination for the past three decades.

U.S. advisers have trained Costa Rican police before, according to U.S. and Costa Rican officials. But the training appears to have reached a new threshold, considering the number of Green Berets here, the goal of the four-month program and the number of Costa Ricans receiving the training. This country long has made the no-Army tradition a pillar of its neutralist foreign policy.

Officials of the Reagan administration for several years have sought to elicit closer and more visible cooperation from President Luis Alberto Monge's government in a regional alliance against Sandinista-ruled Nicaragua, according to Costa Rican officials.

To some degree, they have succeeded. Monge has allowed anti-Sandinista rebel forces to use Costa Rica as a logistics base and he helped the administration lobby recently for more U.S. funding for the guerrillas. His government also has followed the U.S. lead in diplomatic efforts such as the Contadora peace talks.

Military aid has risen from $300,000 in 1981 to $9.2 million this year. It pays for M16 rifles, machine guns, mortars and antitank weapons to equip the new commando force.

At the same time, Monge has been reluctant to allow Costa Rica to be seen as part of U.S. military deployment against the Sandinistas, as Honduras often is, and has proclaimed neutrality as his government's official policy.

Figueres also emphasized that the Green Beret training program was unwise as much for how it might be understood as for what it actually was doing.

"In theory, and under different circumstances, this might have been a good assistance program," he said. "But right now it is inopportune because it may confirm the suspicions of the Nicaraguan leaders that the United States is looking for a pretext to try and overthrow them."

U.S. officials have denied that the new training program marks a departure from Costa Rica's tradition of banning military forces. Both the officials and Monge have cited the danger of subversion from Nicaragua as a reason Costa Rica needs increased military ability.