MANAGUA, NICARAGUA, OCT. 5 -- President Daniel Ortega inaugurated a dialogue today with political opposition groups under terms of the Central American peace plan, as U.S. diplomats said the embassy is adopting a new activism in support of that opposition.

Ortega, the only speaker at the opening ceremonies, asked the 12 opposition groups that attended, ranging from ultraconservative to Marxist-Leninist, to limit the discussions to how best to carry out the peace accord.

Signed Aug. 7 in Guatemala by the five Central American presidents, the plan obliges Nicaragua to end a five-year-old state of emergency and facilitate political pluralism.

Ortega called on the participants to "put aside our philosophical and ideological positions" and "not . . . use the dialogue to provoke a confrontation among party positions." Opposition leaders said they will raise broad issues, such as the separation of the ruling Sandinista party from the state and the Army.

Two previous attempts at dialogue since October 1984 ended in failure.

Friction over procedural details remained, and the substantive talks will not begin until Thursday, Sandinista officials said. Ortega started the dialogue today so it would be at least formally under way when he addresses the U.N. General Assembly Thursday, one of his aides said.

The peace plan calls for a dialogue of "national reconciliation" between the region's five governments and their respective political oppositions. The pact does not call for direct talks with armed rebels, who in Nicaragua are the U.S.-backed contras. Despite mounting pressure, the Sandinista government has rejected such talks.

Meanwhile, U.S. Embassy officials said they have decided to take advantage of a political opening that the Sandinistas have permitted under the plan by taking a higher profile in showing support of opposition activities.

"If Nicaragua is going to become the country the peace agreement calls for, we're going to test the limits," a U.S. diplomat said.

Since mid-August, the State Department has contacted Latin American and European governments asking them to follow the U.S. lead in sending their diplomats to observe Nicaraguan opposition marches and rallies. Some European governments were approached by U.S. diplomats in their home countries, an embassy official said.

Since Aug. 7, the presence of U.S. Embassy observers has been conspicuous at rallies, such as a peaceful gathering yesterday in the Managua suburb of Masaya, where the acting ambassador, John Modderno, put in an appearance.

"We do it to give the opposition confidence. It's Uncle Sam saying, 'We haven't abandoned you,' " an embassy official said. Envoys of many countries attended the Oct. 1 reopening of the opposition daily La Prensa but have been more reluctant to appear at political demonstrations.

Also this week, the embassy will arrange a series of live interviews via satellite for Nicaraguan journalists with U.S. policy makers. In groups of about five at a time, Nicaraguan reporters will be invited to the embassy to question American personalities whom they will see on a television screen hooked up to a government system known as Worldnet.

The embassy did not request permission from Sandinista authorities to install or activate the satellite dish in the embassy compound, U.S. officials said.

"Because there is media freedom now in Nicaragua, it seems like a good opportunity," a U.S. diplomat said. He said reporters from the Sandinista-controlled press would not be excluded from the interviews. To comply with the peace plan, the government reopened La Prensa and the Catholic Radio and suspended censorship that had been in effect before they were closed.

The leftist government is very sensitive about U.S. Embassy involvement with opposition groups. Ortega has said the opposition will be allowed a wider margin of activity only if it pursues nationalist positions and does not become "a tool of Reagan administration foreign policy." Despite the peace accords, President Reagan is preparing to ask for $270 million in new aid for the contras.

Not all opposition leaders welcome the embassy's initiative. Erick Ramirez, head of the moderate Social Christian Party, said, "It doesn't help us at all to maintain too close relations with the U.S. Embassy. We want to keep our politics independent."

Ramirez's party, one of the largest in the opposition, depends on funds from the international Christian Democratic network of parties in Europe and Latin America. It receives no U.S. aid, Ramirez said.

The embassy also has started an attention-getting speakers' program that began Sept. 17 with the one-day visit of Education Secretary William J. Bennett. He infuriated Sandinista officials by advocating support for the contras during his Managua stay. A short list of possible Columbus Day speakers includes the prominent conservative Jeanne Kirkpatrick, a U.S. envoy said.

The embassy has not given any direct financial aid to the opposition, a U.S. Embassy official said, but has provided television and video recorders to five parties.