Heavy fighting erupted yesterday between rebels and government troops in Leon, Nicaraguan's second largest city. Antigovernment guerrillas claimed to have killed more than 400 soldiers in battles throughout the country since launching a major offensive Saturday.

Reliable eyewitnesses said that Leon, which President Anastasio Somoza described Tuesday as under total government control, was in a state of "total chaos."

The witnesses said that while civilian rebels backing the guerrilla Sandinista National Liberation Front were engaged in heavy street fighting with the troops in Leon, other armed citizens began looting and burning in the downtown area.

The guerrilla death figures, listed in a Sandinista communique that put the Front's own losses at 66 deaths and 154 wounded, contrasted sharply with those given Tuesday by Somoza.

While he said he had no civilian casualty figures, Somoza put the losses in the National Guard - which comprises all of the armed forces - at 30 since Saturday. The Sandinistas also said that more than 550 of the Guard's 7,500 members had deserted and joined them in urban battles since Saturday.

There is no way to verify either the government figures, which seem low considering the level of fighting, or those of the guerrillas, which appear inflated. Areas where fighting is heaviest have been sealed off to both the press and the Red Cross by the National Guard.

Sporadic but heavy fighting by guerrillas and anti-Somoza opposition has convulsed areas of Nicaragua for weeks now, but a civil war began in earnest Saturday night. The guerrillas launched simultaneous attacks on police and National Guard installations in five of Nicaragua's largest cities - Managua, Leon, Massaya, Esteli and Chinandega.

While the main of guerrillas apparently quickly withdrew following the attacks, the fight was taken up by armed youths and men with the apparent support of large percentages of the populations.

The local rebels quickly barricaded extensive portions of the cities and street battles with the National Guard began. While in earlier fights, the rebels appeared armed primarily with small-caliber weapons, they now seem to have taken possession of more substantial arms captured in last weekend's attacks.

This was the situation in each of those cities:

While fighting intensified in Leon, witnesses in Masaya, where a major government offensive against the rebels began Monday, said that virtually the entire downtown area has been burned and destroyed.

Although Somoza declared Masaya also to be "under control" on Tuesday, reporters who managed to sneak past government outposts, and refugees fleeing the city, said many of Masaya's streets were still closed by rebel barricades.

They reported numerous bodies. Among buildings destroyed by fire was the government-run National Bank of Nicaragua, where looters were reportedly leaving with large sacks of money.

Large portions of both Esteli and Chinandega in the north were reportedly still under rebel control.

In and around Managua, where at least eight police and National Guard stations were attacked Saturday, the situation was comparatively peaceful but increasingly tense.

Most stores and businesses here are closed as part of a nationwide strike but daytime traffic remained heavy, with large groups of people gathered on corners.

Substantial parts of the city near National Guard installations have been closed to traffic, and motorists yesterday were stopped and searched by soldiers at a number of checkpoints.

At nightfall, activity here ceases. Streets both in Managua and in rural areas are empty, and there is an echo of distant gunfire. Many people say they are afraid to leave their houses. Each of the four cities now under siege have reported severe food and medicine shortages and many areas outside Managua are without water and electricity.

In government-controlled sections of Masaya yesterday, and in other towns including Diriamba and Jinopepe in the south, National Guard troops yesterday reportedly were moving door-to-door, arresting all young men.

Most opposition political and civil leaders are in hiding, and a number have been arrested. At least one, Gustavo A. Arguello, of the Nicaragua Democratic Movement, has died in jail. Instructed by the National Guard to pick him up at the jail yesterday, Arguello's brother arrived to find a corpse. The Guard said he had died of a ruptured ulcer.

As opposition politicians met virtually around the clock, a Sandinista communique of Tuesday demanded that the U.S. government take recognition of the fact that "there is still time to avoid a bloodbath in our country."

The Sandinistas said the "Somoza is and has been sustained with arms, money and American mercenary assistance by the Washington government.

Although the United States has canceled most future aid to the National Guard, it is well-stocked with American-made aircraft as well as armored vehicles and rifles.

[In Washington, the U.S. government announced support for Venezuela's request that foreign ministers of the Organization of American States meet to seek a mediation role. There was as yet no agreement on when such a meeting should be convened.]