A massive car-bomb explosion at a crowded market in Syrian-controlled Baalbek in the Bekaa Valley killed 35 persons and injured more than 100 today in the war-torn country's third serious bombing in three days, police reported.

The attack came as U.S. special envoy Robert C. McFarlane met for almost six hours in Damascus with Syrian President Hafez Assad but failed to break the deadlock over withdrawal of foreign troops from Lebanon.

McFarlane termed his talks with Assad "a very useful dialogue," but admitted that he had achieved "nothing concrete" on the key issue of the continuing presence of Syrian troops in Lebanon, Washington Post correspondent Loren Jenkins reported from Damascus.

No sooner had McFarlane left the presidential palace after his talks with Assad and headed for Saudi Arabia--the next stop in his six-nation tour of the Middle East--than a Syrian Foreign Ministry official said that despite the visit "there has been no change in the Syrian position."

"Syria is determined in its stand against the agreement of submission," Syrian radio quoted Assad as telling McFarlane, referring to the Lebanese-Israeli troop withdrawal agreement, which is dependent on Syria removing its forces as well.

The death toll from the car bomb in the ancient city of Baalbek, 35 miles east of Beirut, was the highest in any explosion in Lebanon since April, when the U.S. Embassy was destroyed by a bomb that killed at least 60 people. On Friday, a car bomb outside a crowded mosque in Syrian-controlled Tripoli killed 19 persons and that night a bomb in a Christian neighborhood of Beirut killed one person and injured five.

General lawlessness has grown, and Lebanon's security situation has deteriorated in recent weeks as various Moslem and right-wing Christian factions have stepped up their attacks on each other despite the presence in the country of Syrian and Israeli occupation armies and a multinational peace-keeping force.

The car bomb in Baalbek, estimated at 220 pounds of TNT, left a scene of dead and maimed shoppers and rubble of autos, pavement and vegetable stalls at the main market of the city of 25,000. It blew out windows of buildings half a mile away.

Shiite Moslem militiamen sealed off streets, occasionally firing shots into the air to clear the way for ambulances. Hospitals appealed urgently for blood donations.

A man claiming to represent a shadowy group called the Front for the Liberation of Lebanon from Foreigners claimed responsibility for the bombing in a telephone call to a foreign news agency here. Callers claiming to represent the group have made similar calls after other bombings, but there is no other proof that such a group exists.

Baalbek, which is in the northern part of Lebanon's Bekaa Valley, near the Syrian border, has been under Syrian control since 1976. It is a predominantly Shiite Moslem city with many supporters of Iran's Ayatollah Ruhollah Khomeini.

There is a large Palestinian refugee camp on the edge of the city, where all the factions of the Palestine Liberation Organization are represented. Although some of the PLO's factional battles have occurred on the edges of Baalbek, the town had been relatively peaceful in recent months.

Jenkins reported the following from Damascus:

The only positive thing to emerge from McFarlane's talks with Assad appeared to be a commitment for Washington and Damascus to continue their dialogue that had broken down last spring when Assad refused to continue talking with McFarlane's predecessor, Philip C. Habib.

"We had a very useful dialogue," McFarlane said as he came out of his meeting with Assad. "It has given us considerable food for thought and a solid basis for continued dialogue."

Syrian officials made it clear that the length of the U.S. envoy's discussions with Assad--five hours and 50 minutes--did not represent any real bargaining. Rather, they said, the time was devoted to a reiteration by Assad in detail of why Syria stood "irrevocably" opposed to withdrawing its forces from Lebanon--estimated at about 40,000--before Israel "unconditionally" removes all of its troops from southern Lebanon.

Although McFarlane's visit had been preceded by reports that he was bringing "new ideas" to break the impasse, Syrian officials indicated that he had brought little new beyond a willingness to listen to the Syrian case.

The official Syrian newspaper Tishrin said in an editorial yesterday, the day McFarlane arrived here from Beirut: "The problem with U.S. policy in the Middle East is that it basically depends on changing means without changing stances or policies . . . . This is obvious as the U.S. envoy begins his mission and carries nothing new except trying to market ugly American policies through rovel means."

After visiting Beirut twice and Tel Aviv once last week at the beginning of his first Middle East swing, McFarlane flew here in hopes of gaining Syrian acceptance of the U.S.-brokered Israeli-Lebanese accord for foreign troop withdrawals from Lebanon.

That agreement stipulated that Israel would withdraw its military forces from Lebanon only in conjunction with a simultaneous withdrawal of Syrian and Palestinian forces occupying the nation's north and its eastern Bekaa Valley.

Syria, which was not a part of the negotiations conducted under Habib, has denounced the agreement because of conditions--such as the right it leaves Israel to control a buffer in southern Lebanon--that it considers both a violation of Lebanese sovereignty and a direct security threat to Syria.

That position had been explained to Secretary of State George P. Shultz when he met with Assad a month ago during a hurriedly arranged Middle East trip. According to Syrian sources today, it was presented to McFarlane again in almost the same terms in his meeting with Foreign Minister Abdul Halim Khaddam yesterday and with Assad today.