President Reagan declared yesterday that the regional peace plan adopted by five Central American presidents "does not address U.S. security concerns in the region" because it does not require the departure of Soviet-bloc and Cuban troops from Nicaragua.

"The Soviet-bloc and Cuban forces must leave," Reagan said in a speech to the Organization of American States (OAS) in which he reiterated plans to press ahead with a request to Congress for $270 million in aid over 18 months for the contras fighting the government of Nicaragua.

In an address that some White House officials said was intended as a conciliatory gesture, Reagan described the regional peace plan sponsored by Costa Rican President Oscar Arias as "a positive movement" and "a step in the right direction," but Reagan insisted, as he has before, that it did not go far enough.

For the first time, Reagan held out the possibility that the contra aid, if approved by Congress, could be converted to humanitarian assistance once democratic reforms satisfactory to the United States were adopted in Nicaragua and the contras are "allowed to contest power politically without retribution."

But in making this gesture toward compromise with Congress, Reagan was not more specific about how aid would be linked to the democratization process, and opponents of contra aid sharply criticized other aspects of the address.

"I think this is nothing more than a continuation of his policy to overthrow the Nicaraguan government," said Rep. George Miller (D-Calif.), chairman of a group devoted to defeating contra aid. Sen. Christopher J. Dodd (D-Conn.), chairman of the Foreign Relations subcommittee on western hemisphere affairs, called Reagan's speech "a fatal mistake" that "squanders the opportunity for peace."

House Speaker Jim Wright (D-Tex.), who sponsored a peace plan with the president last summer, said Reagan was "less strident" in the address yesterday but "it was not what you call a completely conciliatory speech, it was somewhat confrontational." Wright said "there is not any disposition" in Congress "to pass military aid while we negotiate for peace."

The president said that only the plan he sponsored with Wright would eliminate "communist colonialism on the American mainland."

Reagan's address followed a series of moves by Nicaragua to comply with the terms of the Arias plan, including the reopening of the opposition newspaper La Prensa and Radio Catolica.

The president said "we cannot be satisfied with facades of freedom erected to fool international opinion and then quickly dismantled when the pressure is off. We must insist on real democracy in Nicaragua -- not for a week, not for a month or a year -- but for always."

"Democracy doesn't mean opening one newspaper and one radio station -- but opening them all," he said. Reagan also declared that "the Sandinistas have to understand that they do not have the option of being dictators. Their only option is to lead a political party, and serve for limited terms of office, if choosen by the people in free and fair elections."

A senior administration official acknowledged yesterday that "over the last month or two we've been losing support" for the contra aid package and said Reagan will submit it soon and seek a "vote certain" before the scheduled Nov. 21 recess. The peace plan initiated by Costa Rica's Arias calls for a regional cease-fire by Nov. 7, and the senior official said the administration would be "delighted" to convert most of the contra aid to humanitarian aid if there is a negotiated cease-fire by then.

The president described a scenario for such a conversion in his address yesterday after declaring that "continuing aid to the democratic resistance is not only a moral obligation, it is the essential guarantee" that the Sandinista government will undertake reforms. "I will not walk away" from the contras, Reagan pledged.

Once a cease-fire is "fully in effect," Reagan said, "only that support necessary to maintain the freedom fighters as a viable force will be delivered. Then we -- and they -- will be watching to see how genuine the democratic reforms in Nicaragua are.

"The best indicator will be when the freedom fighters are allowed to contest power politically without retribution, rather than through force of arms," Reagan said. "As that happens, our support levels to the resistance forces will decrease proportionately. The assistance money will then be redirected to strengthening the democratic process under way in Nicaragua."

While suggesting such a scenario, Reagan was unstinting in his criticism of the Sandinista actions in recent months. He said that after permitting a rally by the internal opposition recently, the Sandinistas detained 18 of the Social Christian Party members "on trumped-up charges."

"The Sandinistas must learn that democracy doesn't mean allowing a rally to take place and then arresting those who take part," he said. The president also called on the Nicaraguan government to release 10,000 political prisoners, "some of whom have been imprisoned for as long as eight years."

Aryeh Neier, vice chairman of Americas Watch, said the estimate of 10,000 political prisoners, while used repeatedly by the administration, is not supported by the International Red Cross and efforts by his group to count such prisoners. He said the administration is double-counting some prisoners and he estimated the number of political prisoners in Nicaragua at 4,000 to 5,000.Staff writers Tom Kenworthy and Lou Cannon contributed to this report.