Thousands of refugees streamed from the slums of the capital today as President Anastasio Somoza's National Guard turned heavy firepower on Sandinista guerrillas and local youths manning barricades in a showdown in their two-week offensive against the government.

Columns of black smoke rose from the northern edge of the city near the airport, where Guard planes rocketed rebel positions, and vehicles caught in the cross fire burned.

Although the U.S. Embassy called for evacuation of American dependents, the expected Pan American Airways airliner failed to land because of the intense fighting. About 60 U.S. citizens, mostly women and children, gathered at the currently unoccupied ambassador's residence.

Somoza, at a news conference inside his heavily fortified office, again vowed he would not resign. Shouting, his voice shaking with rage, Somoza again accused Panama and Cuba of direct complicity in what he has called an international conspiracy against his government.

The foreign ministers of Venezuela and Ecuador arrived from Costa Rica and were ferried by helicopter from the besieged airport to Somoza's office, where one Nicaraguan official said they had come to press the "urgent need for a short-term political solution" to the crisis.

U.S. official Jack Martin said there are no plans to close the embassy. Others said a C130 transport plane was waiting on standby in the Panama Canal Zone to pick up the dependent evacuees if necessary.

This morning, while the evacuees stood outside the embassy compound, sounds of a fire fight - apparently not directed at the embassy itself - came from nearby. Mothers ran inside with their children, and the entire group traveled by convoy late in the morning to the residence, encircled by a high fence on a nearby hill.

Among the buildings burning in the capital reportedly was the offices of La Prensa, the country's opposition newspaper. The paper's editor, Xavier Chamorro, whose brother Pedro Joaquin Chamorro's murder last year touched off antigovernment demonstrations, said he has been unable to reach the newspaper plant. But he said people living nearby telephoned and said the building was gutted.

Steady streams of Nicaraguan families carrying bundles of belongings walked through the overgrown vacant lots that - since a 1972 earthquake destroyed much of the commercial part of Managua - separate neighborhoods. Most people sought refuge in churches or Red Cross centers away from the fighting.

Managua is crescent-shaped and hugs the southwest shore of a large lake. Away from the waterfront, a four-lane bypass forms the crescent's outer curve.Inside the highway lie most of the city's business districts, middle and upper-class neighborhoods and the central market. Just outside it are widely spaced slum neighborhoods that are home to most of the half million population.

"We've come from San Judas," said one woman, who with her children had joined hundreds already sweltering in a huge Red Cross warehouse. "The boys are in the streets fighting and calling on the people to come out and bang pots and pans, anything, to make noise.

"There is no electricity, no water," she said. "The people are trying to leave because they know the planes will come and they're afraid."

Most of the slums have been organizing for months for what is now happening in Managua, designating civil defense centers and gathering stores of food and medicine. Many residents are believed to be actively aiding the guerrillas and many slum youths are now on the barricades.

Although youths said last week that they were waiting for weapons promised by guerrilla organizers, much of the expected armament apparently never came. Today's battles were marked by the now familiar ping of small-caliber rebel weapons, followed by thunderous Guard counterfire.

Red Cross workers were unable to enter the slums to aid the wounded. Six Red Cross workers have been killed since the civil war began, then temporaily sputtered, last September.

One was shot last week in rebelheld territory in Matagalpa, 80 miles north of here, when he entered the city as part of an ambulance crew to pick up a wounded National Guard soldier. Swiss International Red Cross officials working with the local reflief organization have now prohibited workers from entering combat zones.

Following a regional appeal last week from International Red Cross offices in Rio de Janeiro, the organization has requested a direct appeal from its Geneva headquarters for both sides in the fighting to respect the Red Cross banner and allow dead and wounded to be removed from battle zones.

The International Red Cross has also received an appeal from Sandinista National Liberation Front guerrillas to take possession of what the Sandinistas say are 53 National Guard and government prisoners, including a Guatemalan Army general allegedly captured last week in Leon.

In today's news conference, Somoza produced the drivers' licenses - with photographs recognized by some of those present - and identification cards of three Panamanians he said were killed last week in battles along the Costa Rican border.

One set of documents were those of Hugo Spadafora, a former Panamanian vice minister of health, who last fall quit his job and organized a Panamanian volunteer brigade to assist the Sandinistas.

The brigade, and Spadafora's involvement in it, were publicly known from the outset and acknowledged by the Panamanian government - which has said the brigade has its moral but not official support.

"This exposes to the world Panama's aggression against Nicaragua," Somoza said. He also said the National Guard had captured "more than 300" Belgian-made rifles of a type sent to Cuban dictator Fulgencio Batista in 1958, and presumably later confiscated by Fidel Castro.

"I also accuse Cuba of sending arms" to the Sandinistas, Somoza said.

Somoza has requested that action be taken by the Organization of American States against a number of Latin American countries, including Costa Rica, from which the Sandinista invasion was launched two weeks ago. CAPTION: Picture, Unidentified American woman shields her child as they take cover during the attempted evacuation. AP