The Nicaraguan National Guard intensified its attack yesterday on portions of Matagalpa, 100 miles north of here, following government strafing and bombing of the opposition's stronghold city by light aircraft Tuesday.

Four were reported killed in Tuesday's air attack, with a number of other wounded.

Eyewitness accounts from Matagalpa said there was no count of the number killed and wounded yesterday. They said Red Cross units had been largely unable to enter the city because of a heavy exchange of gunfire between the National Guard and youthful vigilants who are now believed to control at least half of its area.

For the past five days, Matagalpa, with a population of 60,000, has been in a state of virtual civil war. All businesses in the city are closed, and entrance roads are barricaded as part of a nationwide strike called Friday against the government of President Anastasio Somoza.

Tuesday night, the government denied reports that aircraft, apparently light one or two-engine planes of the Nicaraguan Air Force, had been used there. Yesterday morning, however, a spokesman for Somoza confirmed that "snipers" attacking the National Guard had been hit by air strafing and bombing.

Numerous rumors yesterday of an all-out air attack on the city were unconfirmed.

One news photographer who entered the city yesterday was given a letter by the president of the Matagalpa Red Cross to take to Archbishop Manuel Obando Bravo.

"Your presence is urgently requested here to mediate this terrible situation," the hastily typed letter said. "Innocent people are dying and it will continue if the church" does not intervne. "We pray that you won't disappoint us."

The archbishops, who mediated in last week's seizure of the national palace in Managua by left-wing guerrillas, arrived in Matagalpa yesterday afternoon.

Tuesday marked the first time that the Nicaraguan Air Force has been used to combat civilian insurgents in the current round of violence. There were reports earlier this year, however, that small bombs had been dropped from helicopters in at least one conflict in Massays, south of Managua.

According to the International Institute for Strategic Studies in London, the Nicaraguan National Guard has 7,100 men, 5,400 of them in army or police duty, 200 in naval duty and 1,500 in the Guard's air force. There are another 4,000 men in the callable reserves.

The institute says the air force has four World War II B26 twin-engine bombers, six T33 jet trainers, three T28 countersurgency propellor-driven fighters, one Israeli Arava, five C47 twin-engine cargo planes, 10 light Cessnas and seven helicopters. Nicaraguan is also known to have acquired recently several Spanish-built twin-engine troop carriers.

Managua itself was relatively calm. Although the government said that only eight percent of local businesses here were cooperating in the strike, at least half appeared to be closed. At a complex of offices that includes the headquarters of the Bank of Nicaragua, employes paraded through the hallways chanting and calling on fellow workers to strike.

Meanwhile, the government's Liberal party launched a strong verbal attack in Congress against Venezuelan President Carlos Andres Perez, who was charged with interfering in Nicaraguan affairs by his strong support this week for the anti-Somoza opposition.

A Venezuelan television reporter, who had a sharp exchange of words with Somoza at a press conference Tuesday over the extent of personal and political liberty in Nicaragua, left the country after he said his life had been threatened.

The reporter said he was called by Somoza aides immediately after the press conference and told that his "safety here could not be ensured" for more than the next 24 hours.