A car packed with powerful explosives blew up in a street jammed with shoppers in Christian east Beirut today, killing 27 persons and injuring more than 100, but missing an office of the Phalangist Party that apparently was the target.

The explosion came amid mounting Christian concern over a Syrian military buildup in the hills above Bikfaya, the hometown of President Amin Gemayel, and stepped-up political pressure for Gemayel's isolation or departure from office.

Christian discontent with a Syrian-engineered peace plan led to the ouster of Elie Hobeika, the leader of the Christian Lebanese Forces militia who had approved the plan, and has plunged Lebanon into a deeper crisis.

No one claimed responsibility for today's car bomb, the first this year.

Authorities estimated that 550 pounds of explosives were in the vehicle parked on an east Beirut street when it was detonated by remote control in midmorning. The explosion ripped away the facades of seven buildings and set dozens of cars ablaze.

Shops and stores on both sides of the street were reduced to smoldering, twisted metal and debris, with victims strewn about. Motorists were incinerated in their cars and two Lebanese soldiers passing by in their jeep were burned to death.

"This is not hell, but a street in Beirut," a television commentator said, introducing pictures of flaming cars in a thick cloud of smoke.

A branch office of the Phalangist Party on the second floor of a building 30 yards away was still standing, but its windows were blown out and furniture was overturned.

"God loves us, that is why we were not killed," said a Phalangist militiaman guarding the office.

A woman living in the building, dazed by the explosion, cursed the militiamen, screaming: "They fight and we get killed! They use car bombs and we get killed!"

Observers said that besides attacking the Phalangist office, the bombing may have been intended to fuel resentment against a political leadership still unable to agree on an equitable redistribution of traditional prerogatives after almost 11 years of civil war.

Phalangist Party leader Elie Karameh declared that attempts to destabilize the Christian community would not undermine its solidarity. The Lebanese Forces, who called off a press conference at which they were scheduled to discuss last week's bloody battles between rival Christian factions, said the arms of those responsible for the bombing would be severed.

Elias Saba, a former Cabinet minister and a Greek Orthodox Christian who had just returned from consultations with Syrian leaders in Damascus, corroborated reports that Syria plans to create a political pressure group of Moslem and Christian figures opposed to Gemayel. "The Syrians no longer have any confidence in Gemayel, but it is not easy to get rid of a president," he said.

Syria's military reinforcement in areas surrounding the Christian heartland in the past 48 hours have fueled Christians' fears that a major offensive is planned against them to impose the peace plan. Syrian troops, Soviet-made T54 tanks and rocket launchers were brought to the slopes overlooking Bikfaya, site of the summer presidential palace and other mountain resorts.

Syrian ammunition and armor have been supplied to leftist and Moslem militia groups mobilized near other Christian areas.

Gemayel has said he was not adequately consulted on the peace agreement signed last month by commanders of Lebanon's three main militias -- Shiite Moslem, Druze and Christian. He also cited opposition by Christian traditionalists to reforms in the plan that would curb the powers of the Christian president.

The accord also calls for close cooperation with Syria.