Lebanon is stepping up security ahead of two by-elections, one a key test of pro- and anti-Syrian support among the deeply divided Christian community.

Polls open on Sunday to replace two assassinated anti-Syrians, Walid Eido and former minister Pierre Gemayel.

Former President Amine Gemayel is vying to take his son's parliamentary seat.

His supporters clashed with those of opposition leader Michel Aoun last week. Both are potential candidates in this year's divisive presidential race.

The ruling Western-backed, anti-Syrian majority is expected to easily win the election to replace Mr Eido in mainly Sunni West Beirut, but the result in the Maronite Christian stronghold of Metn is keenly awaited.

Tension has been rising and the army had to intervene there to stop a brawl among rival supporters.

The candidate for Mr Aoun's Free Patriotic Movement (FPM) in Metn, north-east of Beirut, is Camille Khoury.

Mr Gemayel and his allies accuse Syria of orchestrating the killing of Pierre last November and other anti-Syrian figures including Mr Eido, who was killed in a bombing in June.

Opening salvo

"If Gemayel fails [in Metn], he will lose any chance for the presidential elections... and if Gemayel wins, he will kill any ambition for Aoun to become president," said Gemayel aide Antoine Nasrallah in remarks to AFP.

Mr Aoun's FPM won a vast majority of the Christian vote in 2005 parliamentary polls, but his support slipped when he allied himself to the pro-Syria Shia Muslim movement Hezbollah.

The by-elections represent the opening salvos in a campaign for parliament to elect a successor to controversial pro-Syrian President Emile Lahhoud, whose term ends later this year.

The anti-Syrian camp has the simple majority in parliament to elect its own president, but they need the co-operation of the pro-Syrian opposition to reach a two-thirds quorum.

The powerful presidency is traditionally held by a Maronite Christian in Lebanon's sectarian political system.

Mr Aoun, Hezbollah and other opposition groups quit a unity cabinet last year and have been boycotting parliament in a campaign to demand a cabinet veto, after anti-Syrian factions won power in 2005 following years of political and military control by powerful neighbor Syria.

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Michel Aoun