Soviet troops remained in firm control today of all strategic crossroads and important government buildings in Afghanistan's capital city of Kabul, travelers from there reported here tonight.

While the Soviets are patrolling the streets in armored personnel carriers, their machine guns ready, the Afghan Army has stayed in its barracks or stood guard at foreign embassies, a French traveler said. He was a passenger on the first plane to land here from Kabul since the Soviet Army took a leading role in replacing Hafizullah Amin Thursday night with a more pliant pro-Soviet ruler, Babrak Karmal.

Tonight's Ariana Afghan Airlines flight to New Delhi, the first since the reopening of the airport in Kabul to civilian traffic after the overthrow, brought passengers with the first eyewitness accounts of the coup.

According to the French traveler, who declined to give his name or occupation, "The coup was very well prepared, very fast."

He said its start was signaled by a bomb blast at 7 p.m. Thursday, which another traveler. Nick Thomaidis, 17, of Sydney, Australia, said occurred just outside his hotel in a main square of Kabul. The blast blew out part of the telephone and telex building located on the square.

"The first boom was followed by the tank attack on the radio station," said the Frenchman, whose house in Kabul overlooks that spot.

"The bomb blast started everything," added Thomaidis, who was in Kabul to change planes but was stuck there because of the coup.

"It was the signal. It was all quiet before, but all the tanks started moving after that," he said.

The Frenchman said he was in his house when the firing started. "I closed the windows, turned out the lights and made no noise so as not to get shot," he said. "What I saw was a tank burning in front of the radio station. I heard machine guns and guns from tanks."

He said he could not see who was doing the fighting, though diplomatic reports received here said Soviet troops took a combat role, especially around the radio and television station, which was heavily guarded by Afghan soldiers loyal to Amin.

Of the seven tanks that were guarding the radio-TV station, the French traveler said four or five left during the battle and two were destroyed.

The whole battle took just 3 1/2 hours. "By Friday morning," he said, "all was finished, all was in order."

The French traveler estimated there are between 3,000 and 5,000 Soviet troops in and around Kabul, although military roadblocks prevented residents there from moving all over the city.

He said hundreds of Soviet military vechicles were parked at the airport this afternoon and Soviet troops had set up a tent city there. "It looked like a real army, an army on a campaign," he said.

He said he saw only one sign of casualties -- an ambulance speeding up to a Soviet plane.

B. Panda, an Indian businessman, said civilian volunteers wearing white armbands assisted the Soviet troops in patrolling Kabul.

According to diplomatic reports reaching here, these civilians are members of the Parcham (Banner) faction of the Afghan People's Democratic Party, which was put in charge after the Soviets ousted Amin's Khalq (People's) wing of the pro-Moscow party.

The Soviet troops appeared to come from European Russia, not Asian areas of the Soviet Union where residents resemble Afghans. There had been reports that the Soviets had sent troops from their Asian units.

The Frenchman said residents of Kabul appear to have divided feelings about the coup. On the one hand, he said, they are glad Amin, whom he called "an orge" is dead, but they are sorry the Soviet have moved in.

"They feel the new government is worse than the former government because it brought the Russians in with it. But the people are glad Amin has been killed," the Frenchman, added. He said he based his opinion on talks with Afghan friends during the past two days.

He said the new Afghan leader, Karmal, had not yet appeared publicly in Kabul, nor had he been seen on television.

Karmal was reported to have come in from Moscow on one of the first flights of Soviet troops this week. He had been living in Eastern Europe after being purged by Amin and Nur Mohammed Taraki, the former president who was overthrown and fatally wounded by Amin in September.

The Soviets airlifted troops and military hardware at a rate of two planes a minute, the Frenchman said. "It was like a Berlin airlift."

He said he could see dozens of planes, landing every 30 seconds last Wednesday.

Diplomats here speculated that the Soviets told Amin they were sending in troops -- U.S. officials estimate the number at about 5,000 -- to help him fight the rebellious Moslem tribesmen who control much of the countryside. h

Karmal pledged, in a broadcast monitored in Islamabad, Pakistan, to be "an active and a true friend of the world Moslem movement."

The rebels, however, promptly assailed Karmal's government as being "more pro-Moscow than the Amin regime," according to reports from the Pakistan capital.