(Edits scenarios after presidential statement)

Nov 23 (Reuters) - Lebanon steps into the political unknown on Saturday when pro-Syrian President Emile Lahoud leaves office with no agreement among divided leaders on who will replace him.

French-led mediation failed to reach a deal between pro- and anti-Syrian leaders on Lahoud's successor. The outgoing president ordered the army to take charge of security, saying the country ran a risk of descending into a state of emergency.

Parliament had been due to convene on Friday to elect a new head of state but the opposition boycotted the session, denying the 128-seat chamber a two-thirds quorum. The vote has been postponed for a week until Nov. 30.

Here are some scenarios on how the political crisis could unfold.

RIVALS CONTINUE TO SEEK CONSENSUS PRESIDENT

The Hezbollah-led opposition and the Western-backed governing coalition chose to contain the crisis by holding more talks aimed at agreeing on a new president. This would leave the post of president vacant until a deal. The opposition would hold off taking any action while the rivals seek a president acceptable to both sides.

Parliament Speaker Nabih Berri, a leading opposition figure, called for a new session on Nov. 30, showing not all bridges had been burnt between both camps. Discussion would likely focus on new names for a president, who has to be a Maronite Christian under Lebanon's sectarian power-sharing system.

The governing coalition says Prime Minister Fouad Siniora's cabinet should assume presidential powers in the absence of a new head of state. That view is supported by Western and some Arab states. But the opposition has disputed the legitimacy of the Siniora government since all of its Shi'ite Muslim ministers quit a year ago. The opposition rejects Siniora's right to assume Lahoud's powers. Government will remain paralysed.

Maronite leaders will protest against leaving the post vacant, arguing that it undermines Lebanon's Christians.

GOVERNING COALITION MOVES TO ELECT PRESIDENT UNILATERALLY

Some members of the governing coalition say the majority has the right to elect a new president without two-thirds of the legislators in attendance. On this basis, the governing coalition could call its politicians to gather to elect a president. The coalition has an absolute majority of three.

The election would have to take place outside parliament because only Berri has the authority to call sessions in the chamber. The opposition has said such a move would be tantamount to a coup. It would respond, but has yet to declare what it would do. Opposition sources say such a move could lead to large-scale confrontations on the streets.

PROSPECTS OF VIOLENCE

The rival sides have accused each other of arming and training followers and the United Nations has expressed concern that they have been preparing themselves in case of a constitutional vacuum. Many Lebanese fear a further escalation in the tension would quickly spill into the streets. The army has warned against violence and deployed to guarantee security. (Writing by Tom Perry in Beirut)

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