Predominantly Moslem West Beirut came under heavy shelling for the first time in five years today in mounting violence rivaling the worst days of Lebanon's 1975-76 civil war.

At the same time, Israeli warplanes caused heavy loss of life and property in their second straight day of raids against Palestinian guerrilla positions in southern Lebanon.

Despite conflicting reports from the belligerents on Lebanon's other main front in the mountains northeast of Beirut, the Syrians and their Lebanese allies apparently were in effective control of the so-called "Frenchmen's Room," a promontory atop Mt. Sanin overlooking the Christian heartland down to the Mediterranean. But fighting was reported under way for control of Mt. Mazar, about two miles southwest of "Frenchmen's Room," where Lebanese Army troops were reported to have taken up positions in apparent preparation for helping the Christian militias against Syrian Army troops.

In Washington, the State Department said a takeover by Syrian forces of key high points in the mountains of Lebanon has brought a "major change in the status quo" between the warring parties. The department stopped short of criticizing Syria's move into areas previously held by Christian militiamen, but cited it as one of the "serious developments which concern us."

[Spokesman Dean Fischer said the United States has been in touch "with all parties and governments" involved to express American concern.]

The mountain fighting and the rising tempo of shelling in Beirut appeared to compromise official hopes of a rapid end to the nearly month-long round of fighting. Despite yet another cease-fire announcement late tonight, there were suggestions that Foreign Minister Abdel Halim Khaddam of Syria might cancel a visit planned for Tuesday aimed at negotiating a resolution to the crisis.

The shelling of West Beirut was described by the official Beirut radio as indiscriminate. It apparantly was retaliation for earlier shelling attacks on Christian centers.

Heavy mortars of the Morabitoun, the main Moslem militia allied with Yasser Arafat's Palestine Liberation Organization, had lobbed at least a dozen rounds last night at the Mediterranean port of Jounieh, about 10 miles north of Beirut, where many of the capital's Christian residents have fled to escape Syrian shelling in the last 3 1/2 weeks. More shells landed today near Jounieh and Bkerke, the seat of Patriarch Antonius Boulos Khoreiche, spiritual leader of the Maronite Catholics who are Lebanon's largest Christian denomination.

Largely deserted Christian East Beirut also was subjected to heavy shelling from Syrian gunners today. A Christian militia spokesman said seven civilians were wounded in the barrage.

Among the many West Beirut neighborhoods shelled was that housing the office of the grand mufti of Lebanon's Sunni Moslems. One round was reported to have hit his office, Hospital sources said at least two persons were killed and 35 wounded in the day's shelling.

Spokesmen for the right-wing Christian Lebanese Forces militia denied its men were responsible. They insisted the Lebanese Army was firing back at a Syrian Army fire base in the 32-story Murr Tower, an unfinished office building near the unofficial line dividing Beirut into Christian and Moslem sectors.

The renewed Israeli air raids coincided with reports from Israel that Prime Minister Menachem Begin's government had warned Syria through diplomatic channels not to step up its current offensive to wrest control of stretegic mountain ridges from the Christian militia.

[Against this background, the Israeli Embassy in Washington delivered a message from Begin to Secretary of State Alexander M. Haig Jr., Reuter reported. Observers recalled that the United States has been the conduit for messages and warnings between Israel and Syria during past periods of tension in Lebanon.]

The Christian leadership has been sending out recent signals that it is disappointed in the level of Israeli help since the battle for the eastern Christian city of Zahle began April 1. Only today, Pierre Gemayel, the 76-year-old leader of the Phalange Party that fields the most important of the Christian militias, said, "Israel is the only beneficiary of what is happening -- Lebanese killing each other."

This condemnation of Israel followed what Lebanese consider the highly unusual choice of the Rev. Boulos Naaman as the militia's envoy last week to Damascus, where he is credited with having worked out the framework of the negotiations due to start here Tuesday. Father Naaman, superior of the powerful Order of Maronite Monks, is a close friend of overall Christian militia commander Bachir Gemayel, son of Pierre, and a major architect of the militia's controversial alliance with Israel.

His negotiating role in Damascus was widely interpreted as a signal to Syria of the militias' willingness to play down, if not necessarily end, these ties with the Jewish state.

Striking today at multiple targets around four cities over a 40-mile stretch -- from Damour just 12 miles south of here to the port of Tyre near the Israeli border -- Israeli warplanes bombed, rocketed and strafed for more than an hour.

Witnesses in Sidon described the raids there as heavier than yesterday's, in which 18 persons were reported killed and 39 wounded. Beirut radio said first reports indicated at least 25 civilians were killed or wounded today. Later reports from the provincial governor's office in Sidon said at least 40 people were killed or injured in the Mediterranean city alone.

Among the targets attacked were areas around the inland market town of Nabatiyeh, a Palestinian guerrilla stronghold, and the Miyeh Miyeh and Bourj al-Shemali Palestinian refugee camps located, respectively, near Sidon and Tyre.

As has become standard procedure, the Israeli jets dropped thermal balloons to deflect the Palestinians' heat-seeking SAM7 ground-to-air missiles that can be fired from the shoulder.

[An Israeli Army spokesman said the Israeli jets struck a PLO maintenance facility at Miyeh Miyeh, one mile east of Sidon, and a Popular Front for the Liberation of Palestine rocket-launching and antiaircraft base at Wadi al-Akhdar, one mile east of Nabatiyeh, Washington Post correspondent William Claiborne reported from Jerusalem. Other Israeli aircraft hit positions east of Tyre and a guerrilla tank base raided in the air strikes yesterday, the Army said.]