Panama and Venezuela, which have supported U.S. policy in Central America, said yesterday that the Reagan administration's plan to conduct large-scale military maneuvers there for the next five months is counterproductive and ill-timed.

Their response was echoed in statements by several Democratic presidential candidates and members of Congress, who said the increased U.S. military presence would aggravate tensions when a number of Latin American nations are intensifying regional peacemaking efforts.

"I don't think the timing could be worse," said Rep. Michael D. Barnes (D-Md.), one of the congressmen who backed President Reagan's creation of a bipartisan commission on Central America policy. Sen. Alan Cranston (D-Calif.), who is running for president, accused the Reagan administration of relying on "gunboat diplomacy."

But some other members of Congress welcomed the maneuvers, saying they would warn the leftist governments of Nicaragua and Cuba of U.S. interest in Central America and might pressure the Sandinista regime in Nicaragua into negotiations to limit leftist insurrection in the region.

Reagan administration officials have described the planned series of joint manuevers with the U.S.-backed government of Honduras and sea and air exercises off the coast of Central America as a way to intimidate Nicaragua, slow the passage of arms to it from Cuba, and demonstrate support for U.S. allies in the region.

But Panamanian Foreign Minister Juan Jose Amado said yesterday that the exercises will "cause concern and tension" and "will hinder" the attempts at regional negotiations by the "Contadora" group of nations. Named for an island where the group first met in January, it consists of Panama, Venezuela, Mexico and Colombia.

Amado also said his government would not permit the United States to conduct maneuvers from its bases in Panama. A Pentagon official said those bases probably would not have been used anyway.

Venezuelan President Luis Herrera Campins, who also has supported U.S. policy in Central America, praised the efforts of the Contadora nations and said the timing of the U.S. maneuvers is not propitious.

The foreign ministers of the four nations met last weekend and issued a four-point peace proposal, which they urged the United States, Cuba and five Central American nations to support.

"We've virtually slapped the Contadora group across the head," one Reagan administration critic, Sen. Christopher J. Dodd (D-Conn.), contended yesterday.

Former vice president Walter F. Mondale, a Democratic presidential candidate, also criticized the planned increase in U.S. military presence.

"What I'm worried about is that they are increasingly militarizing that situation," he said. "In fact, we need to move ahead with political, diplomatic and other kinds of changes down there using our friends to get a basis for a reduction in tensions and resolution of that dispute."

Reagan yesterday suggested that the exercises are in line with maneuvers the United States often conducts around the world. "I think that there's every reason for us to do so with the responsibility we have in this hemisphere," he said.

"It's certainly a strong message, without a doubt, to Cuba and Nicaragua about their own flow of arms to El Salvador," Sen. Sam Nunn (D-Ga.) said, "and from that point of view it's a message that needs sending."

Rep. Clarence D. Long (D-Md.), chairman of the foreign operations Appropriations subcommittee, said he had mixed feelings about the exercise.

"If it does have the effect of intimidating and bringing the rebels to the table, fine," Long said. "But it may create a provocation to an act that would provide an excuse for us to move in with U.S. combat troops."

Congressional critics also reproached Reagan for claiming support from the Organization of American States, when that group has not taken a position on the Sandinista regime's behavior in Nicaragua. Reagan yesterday said that the Sandinistas have failed to proceed with democratic reforms they promised in 1979, "and the OAS considers that the violation of a contract."

OAS officials said yesterday that the OAS has taken no such position, although it debated the Sandinistas' record last week in a meeting requested by Honduras.

"The president was just misinformed about that," Barnes said. "He's also misinformed about the nature of the 'contras.' "

Reagan said yesterday that most of the "contras"--the anti-Sandinista guerrillas supported by the United States--are former Sandinista revolutionaries trying "to bring democracy to Nicaragua." Barnes said that most of them are former followers of the Somoza dictatorship.

The presidents of the four Contadora nations last Sunday urged that all nations halt the militarization of Central America, engage in dialogue and withdraw all advisers. The Cubans have several thousand advisers in Nicaragua, U.S. officials say, while several hundred U.S. trainers and advisers are in Honduras and El Salvador.

The Contadora effort was followed by a negotiation offer from Sandinista leader Daniel Ortega Tuesday. Reagan yesterday said the offer is a welcome "first step" that does not go far enough.