The Nicaraguan charge d'affaires in El Salvador acknowledged today that "there may be" Nicaraguan mercenary soldiers fighting with rebel Salvadorans against the government here, but he stressed that their existence has not been demonstrated and that his government discourages any such activity.

Meanwhile, heavy fighting erupted throughout the country today in what appeared to be the start of a major new guerrilla offensive designed to disrupt the upcoming elections.

"Nicaraguans are a free people," the envoy, Sandino Rafael Somarriba Guevara, said in an interview. "Any citizen can go and enroll in any guerrilla or government army he likes--Chinese, Japanese, whatever--but this is not to say that this is a Nicaraguan government policy . . . . These are mercenary individuals."

He said it would not surprise him if there are Nicaraguan mercenaries here. "There may be, but they haven't been seen," he said. "I don't know that they exist." He denied that a young Nicaraguan arrested here last month is anything more than a Nicaraguan student at a Mexican university who was on his way home for Easter vacation.

In a separate interview, Salvadoran Foreign Minister Fidel Chavez Mena said the Salvadoran government will give Mexico an official "request for information" tomorrow on that Nicaraguan, Ligdamis Anaxiis Gutierrez Espinoza, who reportedly confessed last month to being a Sandinista guerrilla sent from Mexico to train Salvadoran rebels.

It was the first official response from either government to the strange series of events surrounding the arrest, interrogation and escape of Gutierrez, whom Secretary of State Alexander M. Haig Jr. cited last week as "irrefutable proof" that Nicaragua is sending guerrillas to help overthrow the government here.

Somarriba, the top Nicaraguan diplomat here, insisted that Gutierrez is a 19-year-old Managua native who left Nicaragua Feb. 15, 1981, to study at the University of Nueva Leon in Monterrey, Mexico. "He has never participated in any revolutionary team of ours, neither as combatant, ex-combatant, popular militia or anything of any sort," Somarriba said. "We're a revolutionary country. We don't need to send anyone abroad to study revolution."

Asked about Gutierrez's alleged confession, Somarriba said he would like to know the conditions under which it was obtained.

Chavez Mena took the opposite view, saying Gutierrez's alleged statement to police "appears to be more evidence of the intervention by the Sandinistas in our internal affairs." Gutierrez "was grabbed" by people in the Mexican Embassy March 1, when the Salvadoran police took him there to spot an alleged collaborator, according to the Salvadoran government. The Mexicans say he ran in with two Salvadorans chasing him. He is still there and has asked for political asylum.

Gutierrez allegedly told police that he knew of four guerrilla training camps in Mexico. "We are analyzing that allegation," Chavez Mena said. Somarriba denied the camps' existence.

As promised by guerrilla leaders interviewed this weekend in Nicaragua, heavy fighting erupted all over El Salvador today in what Chavez Mena called part of a coordinated leftist effort to halt upcoming elections. The guerrillas briefly captured the center of the provincial capital of San Miguel, including the main cathedral, which was riddled with bullets. The government claimed to have killed 30 guerrillas and lost three soldiers there.

"We were ready for them," said Lt. Col. Jaime Ernesto Flores, commander of the San Miguel garrison. "The town is now under complete control."

There was also heavy fighting in and around San Vicente and San Sebastian, and the guerrillas briefly took control of the town of Torola, in northeast Morazan Province.

Chavez Mena said the guerrillas now appear to have three goals: to impress Salvadorans with their strength and determination to halt the March 28 elections; to make the government look weak and gain international recognition for their prowess in the form of political support and a halt in U.S. aid to the government; and to win a military victory.