East Beirut, the mostly Christian area of the capital, looked deserted as black smoke billowed from tires set afire by militiamen and soldiers seeking to block the streets. Businesses, schools and universities were closed in observance of a general strike called in support of General Aoun's stand against the agreement, which was worked out during 23 days of talks in the Saudi resort city of Taif.

The accord, which General Aoun described as ''hellish,'' calls for an equal division of the seats in Parliament between Muslims and Christians and an eventual withdrawal of Syria's 40,000 troops in Lebanon to the Bekaa region in the eastern part of the country.

Christian hard-liners led by General Aoun want to preserve the constitutionally mandated Christian dominance in Lebanese politics, despite the growth in the country's Muslim population, and insist that the Syrians withdraw before any reorganization is put in place. 'Syria Out! Syria Out!'

About 15,000 demonstrators holding olive branches, large pictures of General Aoun and Lebanese flags marched at midday to the presidential palace in Baabda, southeast of Beirut, to pledge loyalty to the 54-year-old general, who is barricaded there.

''Syria out! Syria out!'' they chanted. Others burned and defiled the Syrian flag. The 31 Christian members of Parliament who negotiated the Taif pact with an equal number of Muslim legislators were vilified as traitors. ''They sold Lebanon!'' the marchers shouted.

Minutes later, the general stepped out of his bunker beneath the wrecked palace, which has been bombarded by Muslim forces during the heavy fighting of recent months, to address the demonstrators.

''You represent the people's will and hope,'' he said. ''Your presence here constitutes the scrapping of the Taif accord. What has happened in Taif is a mistake.'' The general has threatened to punish the Christian deputies when they return to Lebanon from Taif. Aoun May Be Isolated

But General Aoun's loud and angry rejection of the Taif pact was not supported by other centers of Christian power in Lebanon.

A spokesman for the Lebanese Forces, the major Christian militia, said, ''We are not going to take a stand against the Taif agreement until we study in depth the agreement as it was signed on paper.'' The spokesman added, ''The Christians should act in a realistic manner and seriously consider the international and regional stands on the accord.''

With almost unanimous international support for the peace agreement, the clear inference was that the militia may be prepared to endorse the pact.

General Aoun put his 20,000-strong army on alert on Monday in what press reports described as a defensive measure against possible attacks by the Lebanese Forces. Last February, he began a drive against the Lebanese Forces, ending their domination of the Christian enclave in central Lebanon.