Secretary of State George P. Shultz yesterday rejected Nicaraguan President Daniel Ortega's call for bilateral talks with Washington on the new U.S. peace plan for Central America, saying that negotiations to resolve tensions there must involve all the countries of the region.

"I think it is critical to establish that there is no way in which the United States would want to sit down with Nicaragua to decide what is right for Central America," Shultz told a news conference at the State Department.

"That has to be done by all the Central American countries," he added. "So we're prepared to talk with everybody about peace in the region, but it has to be a regional approach."

Shultz spoke a day after President Reagan announced a proposal, worked out in cooperation with House Speaker Jim Wright (D-Tex.), for ending the conflict between Nicaragua's leftist Sandinista government and U.S.-backed contra rebels fighting a guerrilla war against the Sandinistas.

Reagan is expected to make a speech to the nation Wednesday night, in which he will discuss his peace plan.

The plan's main elements, which the administration wants agreement on by Sept. 30, call for a cease-fire, a cutoff of outside military aid, withdrawal of foreign military advisers and steps by the Sandinistas to restore civil liberties and open up the internal political process. In return, the United States would end military aid to the contras and halt other pressures on Nicaragua, such as U.S. combat maneuvers in neighboring Honduras.

Congressional critics of Reagan's Central American policies have reacted warily to the plan, with many calling it a ploy to demonstrate Sandinista recalcitrance and win votes in Congress for more contra aid. The current $100 million in aid runs out Sept. 30.

However, Wright disclosed yesterday that he had received a letter from Nicaraguan Vice President Sergio Ramirez, which the speaker described as "a very positive response to our initiative." Ortega, in a radio broadcast, proposed an immediate bilateral "unconditional dialogue" in Managua, Washington or a third country to discuss the U.S. plan and Sandinista initiatives.

Rejecting this, Shultz noted that the presidents of Nicaragua, Costa Rica, Honduras, El Salvador and Guatemala began meeting in Guatemala City yesterday to discuss a peace proposal offered earlier by Costa Rican President Oscar Arias to deal with all Central American guerrilla insurgencies, including the situation in Nicaragua.

Shultz said the U.S. plan "is intended, and I hope it has this effect, of helping the Central American presidents to find a road to regional peace. That's the object of it, and it was sent to them in that spirit. We think that the right kind of outcome is regional."

The administration has sent its plan to Guatemala, but refrained from sending representatives, out of fear of being seen as seeking to dominate the meeting or deflecting attention from the Central American presidents. However, Shultz made clear that Reagan's special envoy for Central America, Philip C. Habib, is ready to go to the region if the U.S. plan sparks a favorable response.

"We'll just have to see," he said. "But this is a very strong effort put forward in good faith . . . . Phil Habib is one of our most able and experienced diplomats, and he is prepared to be as helpful as he can."

The U.S. plan, although applying primarily to Nicaragua, uses Arias' plan as a springboard, resembling it closely on such points as its cease-fire call, suspension of outside military aid to both sides and internal amnesty and dialogue aimed at democratization and elections.

However, the U.S. version contains some major differences designed to overcome the administration's concern that the Arias plan lacks sufficient safeguards to ensure that Nicaragua will meet its obligations if the United States disarms the contras. For one thing, the Arias plan would cut off outside aid only to insurgent forces.

Although the U.S. plan does not say so specifically, State Department officials said the contras must have a role in negotiating the cease-fire and it must be acceptable to them before the United States stops military aid.

As a further condition of what the officials call "simultaneity," when the cease-fire goes into effect, Nicaragua would have to take immediate democratization steps such as restoring civil liberties, suspending the emergency law giving the Sandinistas extraordinary police powers and establishing an electoral commission for regular elections.