Lebanese Moslem leaders returned from crucial talks in Damascus today indicating they no longer had confidence in the leadership of the Christian president, Amin Gemayel.

The comments by Prime Minister Rashid Karami and former prime minister Selim Hoss, who have tried to play conciliatory roles between the president and his opponents in the Moslem camp, were uncompromising and harsh.

"The regime has proven that it will stop at nothing to abort any peace initiative, and boycott of Gemayel is now de facto," Hoss declared.

A political source privy to the mood in Damascus said the question now was not "whether to deal with Amin or not but how to make him fall."

Against the backdrop of explicit political warnings to Gemayel, both pro-Syrian militia groups and the Syrian Army beefed up their military presence along the approaches to Christian areas in the north, raising fears of a show of force to make Christians accept a new political accord.

An offensive by pro-Syrian militias last Wednesday was halted the next day, but ground movements indicated that the military option had not been ruled out by Syria. An additional Syrian infantry brigade was dispatched to the village of Dhour Chweir, overlooking the Christian Bikfaya, in the last 24 hours, the sources added.

Syrian tanks and 42 rocket launchers reportedly were moved from the Syrian-controlled Bekaa Valley to the upper Metn, another Christian area. A joint military operations room was set up to coordinate activity among the national Syrian Social Party, the Druze Progressive Socialist Party, the pro-Syrian Baath Party and the Lebanese Communist Party.

Also, Syrian troops were said to have been sent from the northern port of Tripoli in the direction of the coastal Batroun region, controlled by Christian followers of Samir Geagea.

Karami, the Sunni Moslem prime minister, called today for the immediate abolition of Lebanon's system of government based on religious affiliation. He pressed the move as a substitute for an accord brokered by Syria last month but shattered by an inter-Christian power struggle.

House speaker Hussein Husseini, Karami and Hoss spent two days in Damascus, where they consulted with Syrian President Hafez Assad and senior officials on future moves following the scuttling of the peace pact and the eviction last Thursday of Elie Hobeika, its main Christian backer.

Karami and Hoss criticized the latest battles in Christian areas and emphasized that Gemayel's isolation was a foregone conclusion. Karami said the resignation of the president would not speed changes but indicated Gemayel was no longer acceptable as head of state.

Gemayel is a member of the Christian Phalange Party. Karami declared that "the presence of parties at the head of the executive may put the state at the service of the party instead of the people."

The agreement, negotiated by Damascus and three of Lebanon's militia commanders last month, would have broadened Moslem participation in Parliament and government and curbed the prerogatives of the president. The present system assures that the president be a Christian Maronite, reflecting that group's numerical strength at independence in 1946.

The recent accord stipulated that the outdated system of apportioning political power would be scrapped following a transitional period.

"The country is in a terrifying crisis, and the president is responsible," Hoss said.

The assault that ousted Hobeika from command of the Christian militias was led by his chief of staff, Geagea, and Phalange fighters loyal to the president. It left about 300 dead and many more wounded.

On the political level, Syria's outlook has been more difficult to pin down but apparently it envisions formation of a broad-based Moslem and Christian coalition grouping Hobeika and former president Suleiman Franjieh. Unconfirmed reports said that Hobeika may be rehabilitated as a Christian opposition leader and based in the central Lebanese town of Zahlah, 30 miles east of Beirut in the Syrian-held Bekaa.

Another scenario is the replacement of Gemayel by a transitional military regime under the leadership of the Lebanese Army's commander, Brig. Gen. Michel Oun.

The Army's 8th Brigade, which defends the Bikfaya front, is 75 percent Christian and 25 percent Sunni Moslem. Bikfaya is just above the president's summer palace and is the ancestral seat of the Gemayel family. Army commandos also have been dispatched to Bikfaya.