Syria delivered at least 42 Soviet-built T54 tanks to the Shiite Moslem Amal movement and the allied Lebanese Army 6th Brigade today as part of efforts to support a Syrian-sponsored truce and security plan in the embattled Lebanese capital.

Witnesses said they saw the vintage tanks rumble toward Beirut's southern suburbs from the Druze-held coastal town of Damur.

It was the first time Syrian tanks had returned to the Lebanese capital since Syrian troops were evacuated in the summer of 1982 after the Israeli invasion.

The delivery of the tanks to Amal, the mainstream Shiite movement, and the 6th Brigade, which is made up almost entirely of Shiite officers and men, strengthened the position of the Shiites in the jockeying for power now going on in Beirut. Both the 6th Brigade and Amal's military branch have been less well-equipped than other military forces here.

Voice of Lebanon, which is the voice of the Christian Phalangist party, said 46 of a consignment of 50 tanks had reached Beirut to help back a new security plan for the city developed by Syria in the aftermath of recent fighting between Palestinian and Shiite militias and the hijacking of the TWA jetliner.

Lebanese Army officers and Amal commanders supervising the delivery at a lot near Beirut airport refused to comment on the reported deliveries.

Zuheir Berro, a key adviser to Nabih Berri, who is chief of Amal and Lebanon's justice minister, confirmed that the newly received armor was to assist in carrying out security measures. But he said the supplies were for the Lebanese Army 6th Brigade and that Amal just helped to transport them.

Upon their arrival, the tanks were boarded by Amal militiamen and driven off as Lebanese Army officers standing with Amal commanders looked on.

A statement by Amal's military branch said the tanks would be used in the "gathering of heavy weapons," which is supposed to take place as part of the plan to disarm rival militias.

The Shiite-dominated 6th Brigade has been poorly equipped compared to the Christian brigades deployed in east Beirut and the mountains northeast of it.

The 6th Brigade and Amal have worked closely since February 1984, when, with Syrian support, they overpowered Christian-led Army units and took control of the Moslem western half of the Lebanese capital.

President Amin Gemayel, a Christian, and Syrian President Hafez Assad decided at a summit meeting in May that Syrian forces stationed in north and central Lebanon would prop up the Lebanese Army when needed to carry out a security plan for Beirut. There are between 30,000 and 40,000 Syrian troops deployed in central and north Lebanon. They came in 1976 to help supervise a truce that was meant to end the civil war in Lebanon.

The move to dispatch an aging fleet of T54 tanks is linked to Syrian efforts to consolidate a shaky truce in Beirut, which took effect on July 16. The agreement to defuse tension between rival Moslem militias called for the withdrawal of gunmen and militias from the streets as an initial step toward a gathering of weapons from all sides.

The arrival of the tanks coincided with the return of Berri and his Druze ally, Transport and Tourism Minister Walid Jumblatt, from talks in Damascus aimed at forging a pro-Syrian Christian-Moslem front that would provide a framework for national dialogue on political reforms.

Fighting across the Green Line that splits Beirut into Moslem and Christian sectors flared overnight leaving two dead and a dozen wounded.