Syrian tanks and troops rumbled into the battered port city of Tripoli today and began supervising the collection of weapons from rival Islamic forces under a cease-fire accord signed in Damascus last week.

The Syrian takeover of Lebanon's second-largest city, 40 miles north of Beirut, appeared to have suppressed the fundamentalist Islamic Unification Movement, which had fiercely defended its strongholds against a thrust by Syrian-supported leftist militias during the past three weeks. More than 500 persons were killed in the three weeks of fighting, but the city was reported quiet today.

There was no indication, however, that the implementation of the cease-fire would lead to the release of three Soviet Embassy officials kidnaped last week by an Islamic group allied to the Islamic Unification Movement.

The captors had demanded a halt to the Syrian-backed offensive, but there have been no confirmed contacts with the group since the cease-fire was signed late Thursday, after another kidnaped Soviet diplomat was found shot to death.

Late tonight, a caller claiming to represent the captors told two western news agencies that the cease-fire in Tripoli was "deceptive" and that the Soviet hostages would not be freed until the Soviet Union and the United States end Lebanon's 10-year-old civil war.

The message urged the Islamic Jihad, a militant group holding six American hostages, to "intensify operations against the Soviets and Americans alike" and not to release its captives.

The calls, however, did not follow the format of previous calls from the alleged captors, raising questions about their authenticity.

There was no further word on the fate of William Buckley, a political officer at the U.S. Embassy here who was kidnaped 18 months ago.

Unconfirmed reports in some Beirut newspapers last week said he had been killed by his Islamic Jihad captors in retaliation for the Israeli bombing of Palestine Liberation Organization headquarters in Tunisia on Tuesday, but there has been no confirmation and no body has been found.

The Islamic Liberation Organization, which claimed to have abducted three Soviet diplomats and the Soviet Embassy physician, had offered to release the survivors in exchange for a truce and a pledge not to attack Tripoli again. The mysterious group appeared to be linked to Tripoli's Islamic Unification Movement, also known as Tawheed.

The unexplained silence has raised suspicions that the Soviet kidnaping and killing were linked to larger regional and international conflicts.

Moslem militia officials helping in the search for the Soviets speculated today that the captors, claiming to sympathize with the plight of "Islamic Tripoli," may have been linked to outside circles bent on weakening Syria's growing influence in Lebanon and harming Soviet interests here.

The Tripoli truce had raised hopes the missing Soviets would be freed but perplexed Soviet Embassy officials said they still had no news.

The truce agreement called for Tawheed and its leftist foes, who had advanced on Tripoli from Syrian-controlled territory under cover of intense bombardment that leveled much of the center of the city, to lay down their arms and hand over their big guns.

Tawheed fighters turned in an assortment of mortars, rockets, antiaircraft guns, rocket-propelled grenades and personal weapons under the gaze of Syrian officers today, but several told reporters that they were holding some back. The cease-fire called for the withdrawal of all fighters from around the embattled city, but some militias were still in position, awaiting their replacement by Syrian soldiers.

Brig. Ghazi Kanaan, chief of Syrian Army intelligence, directed today's activities, but would not tell reporters how many troops were being used. "It is our presence that matters, not numbers," he said, according to United Press International. The Associated Press reported that a battalion of 800 Syrian soldiers with tanks moved into the city at dawn.

According to Syrian military sources, four battalions are to deploy inside the rubble-strewn city. Those moving into Tripoli today came from regions around it. There are about 30,000 to 40,000 Syrian forces in central and northern Lebanon. Under the terms of the accord, brokered by Syria and also mediated by Iran, the Lebanese Army is to be involved in peace-keeping efforts. But no Lebanese soldiers were in sight as Syrians collected weapons.

Syrian tanks were positioned along roads leading to the Mina, Tripoli's harbor area, previously controlled by the Tawheed militia.

Syria originally had its allied militias in the area attack Tawheed on the grounds that, through its control of the harbor, Tawheed was able to smuggle in arms with the assistance of PLO factions opposed to Syria. Tawheed fighters told reporters they would obey a call by their leader, Sheik Saeed Chaaban, to cooperate with a committee in charge of administering the truce.