(Recasts with contacts, adds Security Council)

By Nadim Ladki

BEIRUT, Sept 20 (Reuters) - Rival Lebanese leaders held discussions on Thursday to defuse rising tension a day after the assassination of an anti-Syrian lawmaker threatened to derail efforts to elect a new president.

But political sources maintained that a parliament session planned for next week to choose a successor to pro-Syrian President Emile Lahoud would not yield the necessary quorum for a vote and the election would be postponed.

A two-third quorum in the 128-seat assembly requires a political agreement between the anti-Syria coalition that holds a slight majority and the opposition that includes Hezbollah, supported by Syria and Iran.

The sources said Parliament Speaker Nabih Berri, an opposition leader allied to Damascus, discussed the fallout of Wednesday's assassination of Christian MP Antoine Ghanem with majority leader Saad al-Hariri, who is backed by the United States and Saudi Arabia, in a telephone conversation.

"Things are still under control," a senior opposition source said. But he said it was virtually impossible that an agreement would be in place in time to hold the presidential election on Tuesday.

"Things have not collapsed but more time is needed to ease tension. A compromise is still possible, eventually," the source said. Parliament has until Nov. 23 to elect a president.

Government ministers affirmed their "commitment to holding the presidential elections on time", Information Minister Ghazi Aridi said after an emergency ministerial meeting. "Terrorism will not intimidate us," he said.

The U.N. Security Council called for the election to be held within the constitutional time frame and without foreign interference despite the killing of Ghanem, which it said was a "terrorist attack".

"On the eve of the crucial period of the presidential election any attempt to destabilize Lebanon, including through political assassination or other terrorist acts, should not impede or subvert the constitutional process in Lebanon," a Council statement said.

TRAIL OF BLOOD

Ghanem was the seventh anti-Syrian figure to be killed since the 2005 assassination of former Prime Minister Rafik al-Hariri.

Ghanem's allies were quick to blame Syria for the latest bombing. Damascus condemned it.

His death cut the anti-Syrian coalition to 68 in the 128-seat parliament -- only three above an absolute majority of 65, whittling away at its leverage in the presidential election.

The funeral of Ghanem, who was 64 and died in a car bombing, is set for Friday.

The latest bloodshed drew international condemnation while Lebanese newspapers and politicians said the killing had set back efforts to reach a deal on a consensus candidate to replace Lahoud.

Berri had called parliament to meet on Sept. 25 to elect the new president but the session will not go ahead without a deal between the governing coalition and opposition.

The Shi'ite Muslim speaker also had indirect contact with anti-Syrian Druze leader Walid Jumblatt, the opposition source said, in a rare reconciliatory gesture.

Agreeing on a new president is seen as a step towards ending Lebanon's worst political crisis since the 1975-1990 civil war. Efforts to solve the conflict have been complicated by the rivals' ties to competing regional powers.

Failure to elect a successor to Lahoud could result in two governments -- one backed by the majority and one by the opposition -- and further destabilise Lebanon.

Prime Minister Fouad Siniora has asked the United Nations to add the Ghanem killing to other crimes being investigated by a special U.N. commission, Aridi said. (Additional reporting by Tom Perry in Beirut, and U.N. bureau)

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