Syrian troops tightened their ring around the Lebanese Christian town of Zahle today with tanks shelling from hilltops overlooking it and a blockade of all roads out of the fertile farming region 30 miles east of here.

Syrian units of the Arab deterrent force exchanged fire with the Phalangist-dominated Lebanese forces, militias of Camille Chamoun's National Liberal Party and town inhabitants, for the third day.

The sporadic fighting continued amid meetings between Lebanese President Elias Sarkis, Zahle leaders and commanders of the deterrent force.

Security and Phalangist military sources estimated the death toll among Lebanese civilians and militia at 17, with 30 more wounded.

A Syrian-proposed cease-fire halted heavy shelling this afternoon and state-run Beirut radio said an "uneasy calm fell on the city," in which about 200,000 Christians of the Melchite rite live.

Last Tuesday, Elias Hanash, a controversial National Liberal Party figure, tried to enter Zahle and take over the party's barracks and headquarters. Clashes among local party representatives, Phalangists and Hanash supposters followed, prompting intervention by the Syrian force.

Lebanese military spokesmen charged that Hanash was being backed by Al Fatah, the mainstream Palestinian faction headed by Yasser Arafat, but there was no confirmation.

Friday, the fighting became general between Syrian army regulars and Lebanese Christian factions. Five Syrian soldiers and three Zahle residents were reported killed.

Yesterday, Syrian shelling knocked out Zahle's power station and six people in the town reportedly were killed. Nine Lebanese civilians were kidnaped according to the independent daily newspaper An Nahar.

Zahle, strategically located in the Bekaa Valley, is on the road to the Syrian border and can be a shortcut to southern Lebanon, where the Syrian troops shelled villages in the border strip controlled by Israeli-backed Maj. Saad Haddad last week.