Nicaraguan State Security police arrested four opposition leaders Friday night and yesterday and said they are under investigation for a CIA-organized "plot to promote a terrorist conspiracy," according to witnesses, opposition politicians and government officials in Managua.

Detained are Alberto Saborio, president of the Nicaraguan Bar Association; Mario Rappaccioli, head of the largest faction of the Conservative Party; Julio Cesar Icaza, a member of a coalition called the Democratic Coordinating Group, and Duilio Baltodano of the Social Christian Party.

The arrests were the leftist Sandinista government's response to a public meeting in Guatemala City earlier this week between representatives of Nicaragua's internal political opposition and leaders of the U.S.-backed armed rebels, known as contras.

Four armed and uniformed State Security agents went to Rappaccioli's home with a warrant at about 8:30 p.m. Friday, a maid, Ana Cecilia Diaz, said in a telephone interview from Managua. Saborio was arrested at home yesterday morning in Managua, a neighbor said. Baltodano, according to a spokesman at his party's headquarters, was stopped at Managua's airport as he returned from Costa Rica yesterday morning. Details of Icaza's arrest were not available.

During two days of meetings in Guatemala, the six directors of the Nicaraguan Resistance, the contra alliance, and about a dozen opposition leaders "exchanged views" according to one director, Aristides Sanchez, reached by telephone in Miami. He said they discussed an opposition proposal for three-way peace talks including the government, the opposition and the contras.

In Managua the Interior Ministry, which includes the State Security police, published a communique Friday in Barricada, the Sandinista party daily, charging that the meeting had been to form a "united front" between unarmed and armed opposition groups. This front, the communique said, would undertake "terrorist, conspiratorial actions, taking advantage of the political freedom the government has extended under the terms of the {Central American} peace accord" under discussion by the region's presidents in Costa Rica. It said the plot was part of a broad CIA plan to subvert the goverment.

The communique listed 11 opposition politicians said to have participated in the Guatemala meeting, including the four already detained.

Interior Ministry spokeswoman Capt. Nelba Blandon told Reuter in Managua that President Daniel Ortega, who is attending the regional summit, is aware of the arrests.

In Washington, administration officials, who have feared that the outcome of the summit could jeopardize chances for congressional approval of aid for the contras, were at once dismayed and pleasantly surprised.

"I can't understand why they would make such a big mistake when they're trying to talk peace," one official said.