Syrian troops and Christian militiamen fought fiercy for control of a key bridge yesterday as artillery gunners battled into the night. Christian rightist officials said some Lebanses Army units were joining their military ranks.

The radio station of the Phalange party, which fields the largest Christian Militia, broadcast a general call to arms, appealing to "all Lebanese" to forget sectarian differences and join the fight against "the Syrian enemy."

It also broadcast a communique by "the revolutionary command of the Lebanese Army," which it indicated was formed by Christian units that have split from Lebanon's regular armed forces.

This could not be independently confirmed, but diplomatic observers considered such a development possible given the pressure the Christian population has been under in recent days. if true, they said, the reported defections from any ranks could have grave consequences, possibly widening the Syrian-Christian conflict to include Lebanese leftist and Moslem militiamen who have stayed out so far but who are enemies of the rightist Christians.

The Lebanese Army is still rebuilding after being torn by religious and political factionalism during the civil war of 1975-76. It has remained weaker than the militias and has been unable to impose governmental autuhority in the country.

Syrian shelling of Christain East Beirut and its surrounding suburbs started about 2 p.m. apparently in response to militia efforts to seize the karantina Bridge, a key Beirut River crossing point from the eastern half of the capital to Christian areas of Mount Lebanon and the port of Jounieh just north of Beirut. Syrian troops hold the bridge and two similar crossing points. There was no indication that militia attempts to oust them were successful.

Clouds of black smoke billowed over East Beirut after Syrian gunners hit oil storage tanks in an industrial suburb where militia forces had been massing. Flames leaped hundreds of feet into the air and the night sky glowed for hours.

The resumption of Syrain artillery, mortar and rocket bombardments, which seem to be growing more and more intense, came a day after president Elias Sarkis announced a new "security plan," including replacement of his Cabinet of technocrats with a government of Leganon's traditional political leaders, in an attempt to head off disintegration of the state.

The idea was rejected in most quarters, and observers did not see how it could work since Sarkis himself said it would be based on the "goodwill and full cooperation of all parties."

The only major political leader to support the Sarkis plan publicly is Phalange party chief Pierre Gemayel. Meanwhile, however, the Phalange militia has been taking a tougher line against the continued presence of 30,000 Syrian troops, who make up the bulk of the force set up by the Arab League to keep the peace in Lebanon after the 1975-76 civil war.