Tension over the renewed fighting in Lebanon heightened today as leftist leader Walid Jumblatt was reported to have placed his forces on alert.

There were reports that Syrian forces were shelling the Christian city of Zahle, and clashes broke out in the southeastern suburbs of Beirut between the Lebanese Army and Syrian units.

Jumblatt, a leader of the Druze sect, a group with Moslem roots, met tonight with other leaders of the National Movement, an alliance of Moslem and leftist groups with allegiance to hard-line Arab countries. Among those at the meeting was Yasser Arafat, head of the Palestine Liberation Organization.

"We have no choice but confrontation," the Druze chieftain said yesterday at a meeting with followers in a mountain village southwest of Beirut.

"The battle is coming and the threat [from the Christian camp] is direct and existing. . . . There is no way to escape the battle of destiny," the pro-Libyan daily As Safir quoted Jumblatt as saying.

Jumblatt's tone fueled fears that fierce fighting -- so far localized in clashes between the Lebanese Army in East Beirut and the Palestine Liberation Army (the Palestinian contingent of the Syrian Army) and its Moslem and leftist Lebanese allies along the capital's Green Line -- would engulf the entire capital and spread throughout Lebanon, returning to the civil warfare that claimed more than 35,000 lives in 1975 and 1976.

Meanwhile, U.S. Ambassador John Gunther Dean flew to Amman, Jordan, to discuss the situation in Lebanon with Secretary of State Alexander M. Haig Jr., who is touring the Middle East, and other U.S. foreign policy experts. Earlier in the day Haig said at a press conference in Jerusalem that the Syrian shelling of Zahle was "unacceptable."

Soviet Ambassador Alexander Soldatov was summoned by Lebanese President Elias Sarkis for talks. He evaded a reporter's question on whether the Soviet Union would exert pressure on Syria to stop shelling the Christian market town of Zahle, 30 miles east of Beirut.

The Soviet Union and Syria signed a 20-year friendship and cooperation treaty in October last year. Voice of Lebanon, the private radio station of the right-wing Christian Phalange Party, charged in broadcasts today that Soviet experts were assisting Syrian forces shelling Zahle from the Bekaa Plain.

Soldatov gruffy denied the accusation, saying "This report is false and full of lies."

Phalangist radio said the Syrians had used aircraft to support their artillery in bombarding militia positions in the range of mountains overlooking Zahle. A source from the all-Syrian 22,000-strong Arab Deterrent Force, dispatched to Lebanon in 1976, denied the report.

A military source from the Phalangist war council said Syrian troops were using the Soviet-made Grad missile, rocket launchers and 160mm field artillery in their attempts to push into Zahle and in shelling the slopes of ski areas northwest of the city of Bekaa.

A cameraman who visited the entrance to Zahle confirmed that the Syrians were using rocket launchers and that Christian militia artillery was responding from the hills northwest of Zahle.

The war council source said Christian militias, outgunned and outnumbered by the Syrian regular Army, were using B7 rocket-propelled grenades and 120mm artillery.

Clashes between the Lebanese Army and Syrian forces in Beirut's southeastern suburbs broke out early this morning. The Lebanese Defense Ministry has said that four Lebanese Army men have been killed and 50 wounded in the past two days.

The Lebanese Red Cross announced today that its center in Zahle had been badly damaged by Syrian shelling. Its medical supplies were running out and its medical equipment had been lost and wrecked, one Red Cross source said.

One irate Zahle resident reached by phone said, "We are dying and nobody cares. "Where is the pope?" she screamed into the telephone.

The last six days of violence have taken 150 lives and wounded more than 500 people.

Leftist and Moslem groups called for a nationwide mobilization and a National Movement declaration put the blame for Lebanon's problems on supporters. The movement said it was determined to resist and fight "U.S.-blessed Israeli-Christian designs."