GUATEMALA CITY, AUG. 6 -- Nicaraguan President Daniel Ortega said today he was prepared to open talks with the Reagan administration "immediately" on a new U.S. peace plan for Central America but ruled out making internal democratic reforms as part of the negotiating process.

"We are already practicing democracy," Ortega told a news conference here. Demands for internal democratization "cannot be an object of negotiations between states," he said.

In Washington, Secretary of State George P. Shultz spurned Ortega's call for bilateral talks, saying that any peace settlement in Central America must involve all of the countries in the region. {Story, Page A18.}

Ortega's rejection of U.S. demands for internal changes and Shultz's renunciation of direct U.S.-Nicaraguan talks underscored the serious obstacles facing the peace proposal that President Reagan announced yesterday.

The U.S. initiative was unveiled just before the presidents of Nicaragua, El Salvador, Guatemala, Costa Rica and Honduras opened a two-day Central American meeting to discuss a peace plan proposed by Costa Rican President Oscar Arias Sanchez in February.

As that meeting opened here today, the host, Guatemalan President Vinicio Cerezo, said it would stick with the summit's original purpose of discussing the Arias plan, but that inevitably the presidents would also take up Reagan's proposal. While there are important differences between the two plans, both call on the Sandinista government to guarantee human and political rights designed to institutionalize democracy in Nicaragua.

Arias said in a news conference after Ortega's that peace in Central America cannot be achieved without "true democracy" in Nicaragua. He said a major task of the five presidents was to "define" what they meant by democracy.

The Costa Rican called the new U.S. peace plan "positive," but stressed that the Central American leaders had come here to discuss his own peace plan.

Also at the meeting were Salvadoran President Jose Napoleon Duarte and Honduran President Jose Azcona.

Nicaraguan Foreign Minister Miguel d'Escoto, appearing at the news conference with Ortega, said Nicaragua has been developing democracy since the 1979 revolution that brought the Sandinistas to power. "We don't believe these internal affairs can be the subject of international agreements," he said. Otherwise, he added, Nicaragua would have to present its own ideas of how democracy should work in the United States.

Asked whether the Sandinistas are prepared to negotiate with Adolfo Calero and other leaders of the anti-Sandinista Nicaraguan rebels, known as contras, Ortega said, "We have nothing to talk about with Mr. Calero, because the owner of the circus is Mr. Reagan." He added, "We have to talk with the owner of the circus and not with the clowns."

A block away from the hotel where the summit meeting was being held, the top leadership of the contras gathered in another hotel to make known their own position and, they said, add a "moral" dimension to the Nicaraguan presence here. Extraordinary security was in evidence as hordes of plainclothesmen, blue-uniformed police and troops in camouflaged fatigues stood guard in and around the two hotels.

Pedro Joaquin Chamorro, one of the six directors of the Nicaraguan Resistance, the contra umbrella organization, said Ortega was pursuing a longstanding Sandinista aim of resuming bilateral negotiations with the United States while refusing to deal with the rebels.

Ortega "prefers to try to speak English with President Reagan than Spanish with his fellow citizens," Chamorro said. He asserted that Ortega is "trying to avoid the real substance of the U.S. proposal -- the democratization of Nicaragua."

Chamorro and the other contra leaders present, including Calero, Alfonso Robelo, Alfredo Cesar, Maria Azucena Ferrey and Aristides Sanchez, stressed that the contras must be included in any negotiations for a Nicaraguan cease-fire. They said that although such participation is not spelled out in the U.S. plan, it is implied by the call for a "cease-fire in place on terms acceptable to the parties involved."

"We are ready to negotiate a cease-fire here right now," Cesar said. "The question is whether the Sandinistas are ready."

"We are not going to let a cease-fire be negotiated by any country," Chamorro said, "because it is Nicaraguan blood that is being shed in Nicaragua."

The contra leaders said they were "informed" of the U.S. plan the day before it was announced and indicated that they supported it, with some minor reservations. They said they were not bothered by not having been involved in formulating the U.S. plan, because Washington had a right to make its own proposals in view of its regional security interests. They said they welcomed the bipartisan nature of the plan as something they had long been advocating.

In the Sandinistas' news conference, Ortega, reiterating points made yesterday in Managua, asserted that "we are not rejecting the offer of President Reagan; we are disposed to discuss the proposal of President Reagan." He said his government also has "concrete proposals" to make on a cease-fire and other matters, but would make them directly in talks with U.S. officials and not through the news media.

If the United States rejected the Sandinista offer to resume bilateral talks, which were broken off after a meeting in Mexico more than three years ago, it would mean that the Reagan proposal was a "farce" and a "propaganda ploy" designed to secure additional funding for the contras and possibly lead to American military intervention in Nicaragua, Ortega said.

As the Sandinistas were holding their news conference, a representative of the contras circulated in the room notifying foreign reporters of a news conference scheduled by the contra leadership later in the day.