Lebanese Moslem and Druze militia commanders threatened today to isolate President Amin Gemayel and called for his overthrow, as Syria summoned Prime Minister Rashid Karami and other key Lebanese Moslems to Damascus for consultations on how to mend a fractured peace agreement.

Druze leader Walid Jumblatt and Shiite Moslem leader Nabih Berri were both angered by the ouster this week of Christian militia commander Elie Hobeika, who, with Jumblatt and Berri, had signed the Syrian-brokered accord last month. They blamed Gemayel, leader of a rival Christian faction and an opponent of the peace plan, for intra-Christian feuding that led to Hobeika's resignation.

"Cutting short the president's term in office is the only way out of the impasse," Berri said at a press conference this afternoon. "What is required now of the Army is to rebel . . . and avoid being sucked into Amin Gemayel's war against Lebanon." He charged that Gemayel no longer represented Lebanon's Moslems and said that the next Lebanese head of state should be elected by a popular vote rather than by the parliament.

Although ruling out any future dealings with the Maronite Christian president, Berri cautiously discounted the possibility of a large-scale military offensive against Christian areas. "There is no military initiative by us," he said.

But Jumblatt, whose forces stepped up military pressure against Lebanese Army units in Suq al Gharb, a strategic mountain ridge defending access to Gemayel's presidential palace in Baabda, angrily told reporters that "anyone who deals with the Lebanese regime and accepts compromises is a traitor to the homeland and the nation."

In the hills northeast of Bikfaya, Gemayel's hometown, leftist and Moslem gunners shelled intermittently but made no advances.

The Syrian-sponsored accord signed in Damascus last month calls for phased reforms limiting the powers of the president, increased Moslem participation in government and parliament and close ties with Syria. Gemayel was not involved in its adoption and it suffered a major setback when Christian fighters loyal to him forced Hobeika to surrender and leave the country.

The deadlock caused by the realignment in the Christian camp, much of which is reluctant to forgo long-held prerogatives in favor of what has now become the Moslem majority, revived civil war slogans and widened the rift between Moslems and the traditional Christian leadership.

Christian leaders, seeking to unify ranks in the hopes of achieving a broad coalition under the aegis of the Maronite Christian Church, gave no indication of being concerned about a possible new Syrian military intervention to enforce the peace agreement. Jubilant Christian militia commanders said military pressure would only enhance Christian solidarity.

The Lebanese press, quoting sources close to members of the National Unity Front, a pro-Syrian alliance of Lebanese groups and politicians, said an anti-Gemayel front was in the making. There were unconfirmed reports that Hobeika might be asked to join. Several of his aides, who were involved in negotiating the peace pact engineered by Syrian Vice President Abdel Halim Khaddam have fled the country in the past two days.

In Damascus today, Karami, a Sunni Moslem; house speaker Hussein Husseini; and former prime minister Selim Hoss conferred with Syrian leaders on how to steer Lebanon out of the present crisis. Syria would have liked to see its accord implemented without any complications but appeared to be revising calculations as a result of this week's developments.

Gemayel, elected to a six-year term in September 1982 but virtually eclipsed in the last six months, has reemerged as a key and strengthened figure since his supporters forced Hobeika out of Lebanon this week in fighting that police say killed more than 400 persons. Karami told a pro-Syrian newspaper in an interview published today that the consequences of the "failure could be great and serious." He cautioned that the president may have to leave if his position on the accord remains intransigent.

"The matter is now in President Gemayel's hands. He can define the course of the current crisis," Karami told the Ash Sharq newspaper. "If he does not, there will be different alternatives, among them the abdication of his excellency, the president."

[United Press International reported from Madrid that Foreign Ministry spokesman Inocencio Arias said that Berri pledged to put Spanish officials in touch with the kidnapers of three Spanish Embassy officials in Beirut who were seized Friday by relatives of two Lebanese jailed in Spain.]