Moslem fundamentalists clashed with security forces in the southern Egyptian town of Asyut today in the first reported outbreak of violence since the assassination of president Anwar Sadat.

There were conflicting reports on the extent of the incident. A Foreign Ministry spokesman said it was "not serious," but news agency reports, quoting town officials, said that at least two security installations were occupied by armed insurgents and that government reinforcements had been parachuted into the town.

In Cairo, where the streets remained quiet, President-designate Hosni Mubarak strongly reaffirmed Egypt's continuing commitment to its treaty with Israel and the U.S.-sponsored peace process in the Middle East in his first meeting with a Western official.

In remarks made to reporters after a meeting late last night with visiting West German Foreign Minister Hans-Dietrich Genscher, Mubarak compared the peace process to a moving train and said, "It has to go on . . . the wheels must not be allowed to stop. There is reason for it. The road is clear and the policies we have undertaken must continue."

Mubarak, who was sitting next to Sadat when he died in a hail of bullets Tuesday during a military parade, also provided his account of the grim moments for the first time, and confirmed accounts that the slain leader had initially tried to stand up and salute as four gun-wielding dissident soldiers rushed the reviewing stand.

"I sensed the president standing," Mubarak said. "I stood, too, and to my utter horror and disbelief, I saw a man throw a grenade at the stand and then gunfire broke out. I was hurled to the ground and so was the president, but I could not believe what my eyes had seen."

Mubarak refused to divulge details of the ongoing interrogation of Sadat's three assassins, who were captured alive though badly wounded.

"There is a long story behind all this, but I prefer to wait until the interrogation is completed," he said. He did say the assailants were led by a "Moslem fanatic."

At another press conference, Osama al-Baz, the ministry director-general, was pummeled with questions about the ongoing investigation of those involved in the assassination and the many contradictions in accounts of what precisely had happened. He reported that one of the assailants was dead and three others captured, denying reports that more than four men had been involved in the attack or that there had been a serious breach of security.

Baz said security precautions for Saturday's funeral, which have already resulted in a shortening of the procession route, were "more than adequate" to assure the safety of all visiting dignitaries. "It's not going to be a garrison," he said. "We intend to take all measures to protect our guests without turning it into a police state."

There were conflicting reports today about the fighting between police and security forces in Asyut, which is about 250 miles south of Cairo.

An Interior Ministry statement said that one car filled with Moslem extremists had opened fire on a police station, killing and wounding a number of people.

Police killed one of the attackers, injured several others and captured the band's leader, the statement said. Unconfirmed reports put the death toll as high as 12.

The clashes began shortly after dawn as the Moslem fundamentalists emerged from mosques after completing their prayers on the first day of the Id Al Adha feast of sacrifice.

Asyut, almost evenly divided between Moslem and Christian Copts, repeatedly has been the scene of sectarian strife over the past two years. Moslem fundmentalists are particularly strong on the university campus there.

Mubarak disclosed official plans to bury Sadat Saturday on exactly the same spot where he died. A mausoleum is now being constructed there for the slain president.

A government spokesman said today that Sadat's body will be flown by helicopter from the military hospital where he died in the suburb of Maadi, six miles south of the capital, to the main sports stadium in Nasr City, adjacent to the parade grounds.

From there, his body will be placed on a horse-drawn caisson and the funeral procession will follow a half-mile course to the Tomb of the Unknown Soldier. There, at a spot facing the parade reviewing stand where he was struck down, Sadat will be buried until the mausoleum is ready. The ceremony will begin at 11 a.m. local time (5:00 a.m. EDT), a spokesman said.

Funeral arrangements for the president were slightly altered today to shorten the length of the procession, primarily for security reasons. The funeral prayers will take place at a mosque located on the Maadi hospital grounds, officials said, rather than at the Rabaa el Adaweya mosque in Nasr City.

The issue of security for the funeral, which was reportedly the reason President Reagan decided not to travel to Cairo, was aired tonight at Baz's press conference, which was held at the Foreign Ministry for the hundreds of foreign correspondents who have arrived here following President Sadat's death.

Baz said Egypt "understood" why President Reagan had decided not to come and was satisfied with the U.S. delegation of three former presidents and "the incumbent vice president." However, the Reagan administration has said Secretary of State Alexander M. Haig Jr., not Vice President George Bush, will lead the U.S. delegation.

After reiterating the government's previous statements that only four men were involved in the attack on Sadat, Baz was asked to account for the other three soldiers and the driver who were in the truck used by the attackers. "They were not part of the plot," he replied.

Baz said the driver had been "intimidated" and "forced to stop" by the assailants, one of which was apparently in the passenger seat of the truck.

Baz said one of the captured assassins was a member of the artillery unit that controlled the truck, and that another was a reservist in a second unit. The other two were civilians who Egyptian authorities have said were masquerading as soldiers.

Baz refused to comment on how it would have been possible for civilians to sneak into the parade, saying this could only be determined once the interrogation was completed. He did confirm, however, that the assassins belonged to a "fanatic religious group," although he refused to name the group involved. He emphasized that the assailants were only a "handful of people" and said they had been "misled and misguided by their own interpretations of their religious beliefs."

The most detailed information from the government regarding the assassination became available today when the commander of the Republican Bodyguard, Maj. Gen. Mahmoud Masri, gave an interview to the newspaper Al Ahram. He said there were only four "criminals" and that only one of them was an active member of the armed forces.

He also confirmed that the active Army member had had a brother arrested last month in Sadat's massive crackdown on sectarian extremists and opposition elements.

"All the other three," he said, "did not belong to the armed forces, but the first, a youth in his 20s who had been only a few years in the armed services, facilitated their joining the parade column on the morning of Oct. 6."

Masri also said the units participating in the parade were not supposed to have ammunition and that the assailants had bought their grenades and ammunition in upper Egypt and then brought them to Cairo early on the morning of the parade. Masri gave the following account of the attack on Sadat:

"A military vehicle and a motorcycle went out of order during the parade. So when the criminal vehicle reached the reviewing stand and stopped there, everybody thought it had also developed engine trouble.

"Three of the traitors came down from it," he continued.

"President Sadat thought they would come forward to greet him. He rose to return the salute. At this moment, the fourth traitor, who was still on the vehicle, opened fire from his automatic rifle and hit the president on the spot.

"The bodyguard returned the fire and hit the traitor, who was still in the car. But desite his injury, he lept out and joined his three colleagues in a storm formation: two headed for the center of the stand, one for the right and the other the left."

Masri said one of the bodyguards had thrown himself on the president to protect him, pushing him down on the stand ground. He said bodyguard members had also killed one of the assailants, who had rushed to the edge of the stand and could have killed "all those" around the president.

In defense of the bodyguards, whom film and pictures of the attack show far away from the president, Masri said they had hit the three other assailants, one with six bullets, the second with three and the third with one.

He also said l2 members of the bodyguard had been injured in the attack, two of them officers.

Masri said the attack had lasted about 30 seconds and that President Sadat was evacuated in "exactly one minute." He added that the president was vomiting blood and did not answer the bodyguard who pushed him down.

Most eyewitnesses, including a number of military attaches from Western embassies, believe that more than the official number of four assailants were involved in the operation. But an examination of one film of the shooting available here suggests the three soldiers sitting on the side of the truck farthest away from the reviewing stand may not have participated.

The film indicates that at least at first, the three soldiers did not go into action with the others.

If these soldiers and the truck's driver were not involved, the government figure of four attackers would be accurate.

But the government version still does not explain why security officials turned and fled from the assassins, rather than blocking their route toward the reviewing stand. Nor does it indicate how civilians were able to slip past the tight security at the parade with live ammunition, replace the soldiers picked to ride in the truck and arrange for the vehicle to be in the lane closest to the reviewing stand.

In Tripoli today, meanwhile, former Egyptian Army chief Saadeddin Shazli, leader of an exile group intially connected to the assassination, called on Egyptians to stage antigovernment demonstrations and urged the Army not to resist them.

"We . . . urge you, our struggling people, to turn out in powerful, popular demonstrations asking for freedom and release of detainees," the Libyan news agency JANA quoted Shazli as saying.