Secretary of State Alexander M. Haig Jr., promising to make available detailed evidence within days of external involvement in El Salvador's civil war, announced yesterday that a Nicaraguan had been captured while directing Salvadoran guerrilla operations.

In a Capitol Hill appearance to lobby for a proposed $2.2 billion increase in foreign assistance, Haig continued his sharp attack on Nicaragua, declaring that the Sandinista leadership there would be ousted in a free election.

He also asserted that, despite their public claims to the contrary, Nicaraguan leaders have made "very clear" their right and intention to support revolution in El Salvador and other parts of Central America.

Haig ran into some congressional skepticism about the direction of U.S. policy and the evidence of outside intervention, notably from Rep. Clarence D. Long (D-Md.), chairman of the House appropriations subcommittee that heard Haig.

Long said that in recent visits to seven countries in the Central American area, "I did not get the impression that we were even making a contribution toward a solution" of regional strife. He said he found a belief in the area that U.S. "gunboat diplomacy" is harmful rather than helpful to friendly nations.

Shortly before Haig testified, two legislators announced that 104 House members, including 12 Republicans, have appealed by letter to President Reagan to accept a Mexican offer to negotiate an end to the Salvadoran civil war. Mexican President Jose Lopez Portillo's offer has been accepted by insurgent forces in El Salvador but not by the ruling junta nor by the U.S. government.

The congressional letter to Reagan, announced by Rep. James A. S. Leach (R-Iowa) and Rep. David E. Bonior (D-Mich.) said, "The escalating crises in El Salvador, Guatemala and Nicaragua are reaching a critical juncture and run the risk of involving the United States in a major regional conflagration."

A suggestion by Long of new negotiations with Nicaraguan officials elicited Haig's revelation that "today, for the first time, a Nicaraguan military man was captured in Salvador, having been sent there by the FSLN the ruling authority in Nicaragua to participate in the direction which is so evident of this guerrilla operation from Nicaragua."

Outside the hearing room, Haig told reporters that he would call the captive a guerrilla rather than a military man, that the man admitted working with the Salvadoran rebels and that "he was sent there by the Nicaraguan government to assist in the revolution."

Despite Haig's statement, Salvadoran authorities have publicly presented other Nicaraguans in the past whom they charged with aiding the Salvadoran guerrillas.

Haig, who has refused to make public evidence to back up his charges of Nicaraguan support and control of the Salvadoran rebels, said CIA briefings provided to the intelligence oversight committees of the two houses of Congress confirm "in a very clear, very specific and very unchallengeable way" Nicaraguan and Cuban involvement in El Salvador.

Rep. Edward P. Boland (D-Mass), chairman of the House Intelligence Committee, said the information about Nicaraguan support and supply for Salvadoran rebels was "convincing."

Prodded by Long, Haig promised to make some of the intelligence data widely available to members of Congress, perhaps as early as today. Official sources said intelligence agencies have been instructed to prepare data for public release.

Turning to the internal military buildup in Nicaragua, Haig said that country has built an "unprecedented" army of 25,000 men, more than the combined forces of other Central American states, and has imported Soviet-made tanks, artillery and anti-aircraft weapons.

He said that 75 Nicaraguan pilots have been trained to fly Soviet Mig warplanes in Eastern Europe and that 2,000 Cuban military advisers and 70 Soviet technicians and military advisers are in Nicaragua.

Haig also said U.S. Embassy attaches in Honduras had reported, on the basis of interviews with Miskito Indian refugees near the Nicaraguan border, that Nicaraguans, Cubans and "other Caucasian non-Spanish speaking people that they assume to be Russians" had entered Indian villages before they were recently razed by the Nicaraguan military.