Mohammed is a 26-year-old sales manager for the Tehran office of a West German appliance manufacturer. Three months ago he joined the Cherikhaye Fedaye Khalu, or People's Sacrifice Guerrillas, a Marxist group that believes in armed struggle to turn Iran into a socialist country.

In the three days of urban warfare that has toppled the government, obliterated Shah Mohammad Reza Pahlavi's monarchy and brought Tehran under the control of a motley army of street fighters, Mohammed and his comrades have acquired a sizable arsenal.

And they say they are not about to give up their guns, despite orders to do so from the camp of Ayatollah Ruhollah Khomeini.

This problem, multiplied thousands of times, presents the incoming provisional republican government under Prime Minister Mehdi Bazargan with a crucial test of its authority. In fact, it poses a challenge to Khomeini himself.

According to political analysts here, the future of the new government, and of the country as a whole, may well depend on whether the emerging authorities can collect the weapons that have been seized from military depots and give out -- often by Khomeini's own headquarters -- with such abandon in the past few days.

Although resistance from pro-shah military forces has been crushed with incredible speed, the possession of automatic rifles, grenades, submachine guns and heavier weapons by thousdands of youthful fighters threaten to turn Tehran into another Beirut, where various armed groups are laws unto themselves.

Iranians here feel the situation is unlikely to deteriorate to the Lebanese extreme because the divisions among the population are not nearly as varied or deep.

But the differences appear to be enough to raise the possiblity of future clashes between rival groups trying to assert their power and influence in the current revolutionary atmosphere, analysts said.

Another problem, they said, is that the new Bazargan government may not be able to deliver sufficient economic and social improvements soon enough to satisfy the society's havenots, who were in the forefront of the anti-shah movement and who are now fairly well armed.

The "Khomeini committee," which orchestrated the various assaults on Tehran military bases, police stations, imperial palaces and government installations, insist that the arms distribution is under control.

"One word from the ayatollah will make the people turn in their weapons," said Ghassem Salehkhou, a senior member of the committee's press department.

But other sources say the Khomeini camp may be overestimating the willingness of various street fighters to fall into line, especially if it means turning in their newly acquired guns.

Despite repeated appeals on the state radio -- now called "the Voice of the Revolution" -- for civilians to turn in their guns at Khomeini headquarters, few seemed to be doing so today. In fact, scores of people, mostly youths, crowded around an entrance where names were being taken and guns distributed.

Khomeini committee officials said the weapons were being given back to regular soldiers. If this was the case, however, it was evidently not being done in a very military fashion.

Meanwhile, thousands of street fighters armed with Iranian-made G3 automatic rifles, World War II-vintage U.S. M1 carbines, Israeli and West German submachine guns, pistols, revolvers, bayonets, swords and clubs rode around the capital in private cars, pickup trucks and captured Army vehicles.

They secured military installations and government buildings, controlled Tehran's airport, took a number of Iranians and foreigners prisoners, set up roadblocks to check passing cars and simply paraded up and down the streets with their weapons.

Even the Islamic prayer leaders, or mullahs, were getting into the act. At the rear of Khomeini headquarters, one turbaned clergyman sat atop a truck cradling an old-fashioned pistol in his lap.

A short distance away another mullah was on the receiving end. Taken prisoner on suspicion of belonging to the secret police, SAVAK, he was marched into Khomeini headquarters by two youths at gunpoint.

Some of the street fighters' activities seemed relatively organized. But most were the work of youths enjoying the gun-toting thrill of self-importance and authority.

Brandishing an automatic rifle has quickly become a young man's status symbol in Tehran.

A number of disgruntled citizens called the radio station to complain about youths running around with all kinds of weapons. Several people were reported killed or wounded in accidental shootings.

Still suspicious that the armed forces' capitulation to "the revolution" is all a trick, street fighters at Tehran University gave brief weapons screening to interested civilians.

Fighters like Mohammed are forthright about their attitude toward the Khomeini camp's appeal to turn in weapons.

"I won't give up my guns," Mohammed said. Wearing a Palestinian-style checkered keffiyeh headdress and carrying a G3 automatic rifle, he said he didn't think guerrillas of the fanatical Islamic Mujaheddin-e-Khalq, or People's Strugglers, would do so either.

Mohammed said he himself had acquired five rifles and several grenades during the insurrection when an Air Force base, a nearby arms factory and every other military base or establishment in Tehran fell to the street fighters in rapid succession over a three-day period.

Once small and tightly knit, both main Iranian guerrilla groups have expanded into the thousands since the former government and secret police ceased to threaten them.

The starting suddenness with which the Iranian Army crumbled in the face of its first armed resistance showed the inherent weakness of the shah's power base, political analysts said. The country's top military commanders evidently feared that the Army would break up entirely if it tried to put down the insurrection.

It was hard enough for the armed forces to hang together when demonstrators were shouting slogans and throwing rocks. But when they started shooting guns, the Army apparently did not have the stomach for taking a hard line.

In the end, it was the shah's own dream of making his country a major military power that helped wipe out the last remnants of his monarchy. Many of the civilians who took up arms were former draftees who learned how to use their weapons in the shah's service.