The National Guard said it had retaken the provincial capital of Leon from rebel insurgents yesterday, following heavy air and artillery bombardment of the city.

While journalists for the second day in a row were prohibited entry into the combat zone, refugees leaving Leon yesterday morning said the attack - in which the National Guard withdrew from the city, sealed its exists, and conducted a heavy bombardment lasting most of Friday - resulted in extensive damage and numerous casualties.

The U.S. Embassy here has asked for and received promises of cooperation from Nicaraguan President Anastasio Somoza in evacuating to Managua approximately 1,500 U.S. citizens and their dependents living in Leon and other combat areas in the northern part of the country. Embassy spokesmen said yesterday, however, that no specific actions had been decided on.

At least two U.S. citizens have been killed in Nicaragua during fighting over the past several weeks. One, whose name has not been released, was reportedly shot nearly three weeks ago during violence in Matagalpa. The other was dual national Cesar Amador, 22. U.S. officials have been told by police that he was killed during a guerrilla attack on a Managua police station Sept. 9.

U.S. Peace Corps volunteers in Nicaragua were called into Managua last week and have been flying out of the country in groups.

Residents permitted to leave Leon for the first time in several days yesterday told reporters waiting at a military roadblock that the National Guard had stopped the shelling and entered the city early yesterday morning and was conducting door-to-door searches, arresting young men.

The same type of searches and arrests were carried out following successful National Guard operations in Matagalpa and Masaya during the past few weeks. The refugees said that there were still isolated street battles in Leon.

The Nicaraguan Red Cross yesterday morning sent a 10-vehicle convoy of trucks and ambulances to Leon carrying food, plasma and medicine. The convoy was the first Red Cross attemtp to enter the besieged city since two Red Cross workers, traveling in a clearly marked vehicle, were killed by the National Guard Thursday on the Managua-Leon road.

The International Red Cross director for Central America, Raymond Chevallez, traveled to Leon with yesterday's convoy. Chevallez said he "spoke yesterday with Gen. Somoza, and he gave me full assurance" that the Red Cross would be allowed free entry and that its pickup of dead and wounded would not be interfered with.

Somoza, Chevallez said, "expressed regret" over Thursday's shooting "from both himself and his son." National Guard Maj. Anastasio Somoza III is reportedly directing the military operation in Leon.

A National Guard communique issued yesterday said that "units specializing in combating subversion in urban centers have successfully achieved their objective of recovering the city of leon, bringing peace and tranquility to the Leon citizenry that lived hours of terror in the hands of the Communist hordes."

Although earlier refugees who have been waiting at roadblocks outside Leon for as much as a week to rejoin families stranded inside were allowed to enter the city yesterday, journalists were told they needed a special new pass issued by the government "Office of Law" in Managua.

One Peace Corps volunteer, trapped in leon for the past several days along with a U.S. television camera crew and a photographer, was reported trying to leave the city, but none of the Americans had appeared at the military roadblock by late afternoon.

The National Guard confiscated the film of two other U.S. television crews who had entered the city early yesterday morning before the new pass requirements were enforced.

Late yesterday morning, a company of National Guard troops passed into Leon in convoy, along with a World War II vintage Sherman Tank, the second seen entering Leon. The troops augmented a force estimated at 800 already in and around the city of 80,000.

The new soldiers were greeted happily by those at the roadblock, members of special forces units led by Anastasio III. They cheered when told the city was theirs.

According to one special forces officer, apparently in charge of the roadblock, the stiff military attack on Leon was "the fault of President Carter," because of what the officer called indirect encouragement of terrorists under a banner of respect for human rights.

Carter, the officers said, "talked about human rights, but we have rights, too."

The U.S. role in past and current Nicaraguan events is foremost in many minds here and repeatedly comes up in conversations about the current situation on both sides of the battle. While the National Guard officer blamed Carter for intervention, a refugee several yards pleaded with a U.S. journalist to "tell Carter what is happening here. Tell him to send someone here to help us." The National Guard, he said, "is crazy."

Although the State Department has expressed serious concern over the Nicaraguan situation, and has called on Somoza to accept mediation, its lack of a firm anti-Somoza stand has dismayed and angered members of the opposition.

"The people of Nicaragua are fully aware of the role the United States has played in their history," opposition leader Sergio Ramirez said in an interview last week. "They are conscious of the fact that they are being massacred by an army and arms produced in the United States."

The National Guard requires all officers to have U.S. military training before they are considered for promotion.

Nicaraguans, Ramirez said, "are also aware of the changes in policy under Carter. That is why it is of the utmost importance that the United States clearly defines its position . . . that it takes action to pressure, for example, the multilateral institutions" that loan money to the Somoza government.

The government has called the Leon operation necessary to rout communist terrorists and vandals. But many Nicaraguan society, have called the Leon attack an attempt at genocide. Witnesses inside the city during the attack said that some residents were trapped inside their burning houses and afraid to leave for fear of being machine-gunned by strafing aircraft.

For the last year, a leading opposition businessman said, "Carter has criticized Somoza's human rights violations. This far surpasses any violation we ever imagined."