Clashes between anticommunist Afghan demonstrators and government forces caused hundreds of civilian casualties in Kabul Friday and forced Soviet troops to replace some Afghan units yesterday in enforcing new martial law restrictions, reports from the Afghan capital said.

Soviet helicopters and MiG jet fighters buzzed the capital for the second straight day to discourage demonstrations, and occasional bursts of gunfire were heard, according to the latest diplomatic reports from Kabul.

The State Department said it had received reports estimating that the clashes Friday left about 150 Afghan civilians dead and about 350 injured. The department said there were unverifiable reports of many more casualties.

The State Department was unable to confirm reports quoting travelers from Kabul as sayng that about 4,000 Afghan soldiers mutinied and battled Soviet troops after refusing orders to shoot civilian demonstrators in the capital Friday.

State Department officials also said they had no information about the fate of an American reportedly arrested on spy charges in Kubal Friday. The Soviet news agency Tass said the American, identified only as Robert Lee would be tried before a revolutionary tribunal.

On the diplomatic front, the State Department brushed off Soviet President Leonid Brezhnev's call Friday for international guarantees against any outside interference in Afghanistan. Brezhnev suggested in a Moscow speech that such guarantees would lead to the withdrawal of Soviet forces.

The U.S. response, issued yesterday, in effect told the Soviets to withdraw their forces first as "a necessary condition for the establishment of a neutral and nonaligned government acceptable to the people of Afghanistan."

The State Department also said, "We do not believe there are any external activities which can justify the Soviet invasion of Afghanistan or the continued presence of Soviet military forces in Afghanistan."

In Moscow, a report from Kabul by the Soviet news agency Tass indicated that the unrest was continuing. It said the Afghan government and youths formed into military units were "taking energetic steps to isolate and neutralize the provocateurs and instigators."

Tass blamed the uprising on a plot by the U.S. Central Intelligence Agency. The report made no mention of any action by the Afghan or Soviet armies to put down the disturbances.

Witnesses told reporters in New Delhi that both Afghan and Soviet troops were involved in the efforts to combat rebellious demonstrators and enforce martial law.

Two tanks manned by Soviet troops were parked near the airport terminal as some travelers were leaving yesterday. Other Soviet soldiers were manning roadblocks in the nearly deserted streets and driving Soviet-made T55 and T62 tanks with Afghan Army markings, travelers said, indicating that Soviet military leaders considered Afghan units unreliable.

One French traveler told the Reuter news agency in New Delhi that between 50 and 100 Afghan civilians were killed by gunfire during demonstrations in Kubal Friday.

"I saw some Afghan troops firing over the heads of a crowd of about 200 people carrying green Islamic flags," Reuter quoted him as saying. "I didn't see any Soviet troops involved, but I have reliable reports that the Russians were involved in some gunfights."

Other witnesses said they believed the death toll was higher. One European said he saw the bodies of 100 to 200 Afghans lying in a single Kabul street.

In a dispatch from Kabul, Associated Press correspondent Michael Goldsmith said a Western diplomat who took an injured man to a hospital counted more than 50 bodies lined up on the floor for burial.

Another witness was quoted as saying that he saw six civilians lying dead on a street following renewed fighting yesterday morning.

Reuter correspondent Robert Evans reported from Kabul that, according to diplomats, anti-Soviet demonstrators had attacked two hotels in the city Friday and had injured some foreigners, possibly Soviets.

Evans said that Afghan troops and armed members of the ruling People's Democratic Party were manning roadblocks, but that Soviet forces appeared to be increasingly involved in security operations previously carried out by Afghans.

Travelers arrived in the Indian capital, New Delhi, told reporters they heard some shooting in Kabul yesterday morning, although it was not as heavy as the firing that broke out Friday druing the first major public demonstrations in the capital against the Soviet military occupation of Afghanistan.

In general it appeared that action by Afghan troops and militiamen of the ruling Communist Party, backed by a strong display of Soviet air power, had dampered the mass protests Friday that bordered on a popular-uprising.

However, travelers and news reports from Kabul said that a general strike begun Thursday appeared to be holding yesterday, indicating a continuing mood of reisistance to the Soviet installed government of President Babrak Karmal.

The government declared marital law Friday in response to its stiffest challenge since a Soviet invasion force brought Babrak to power in a bloody coup Dec. 27 that replaced one communist Afghan goverment with another.

The State Department said that in enforcing martial law, government forces had set up roadblocks that cut Kabal in half and effectively cordoned off the old bazaar, rfeportedly one of the main hotspots in Friday's disturbances.

Around midday yesterday, a department spokesman said, Soviet aircraft dropped leaflets over the capital claiming that British, U.S., Pakistani and Chinese agents were responsible for the trouble and remainding residents to respect the new martial law regulations.