An opposition candidate has defeated a government-backed rival in a tense by-election near Lebanon's capital Beirut, the interior ministry says.

The poll is being seen as a key battle for the Christian leadership, ahead of presidential elections next month.

Opposition leader Michel Aoun acclaimed victory for his candidate over rival Amin Gemayel, who has alleged fraud.

Overnight, supporters of the two sides were separated by tanks and hundreds of troops.

The election was one of two being contested to find replacements for two murdered anti-Syrian MPs.

Christian cabinet minister Pierre Gemayel was shot dead in November, and Sunni Muslim lawmaker Walid Eido was killed in a Beirut car bomb in June.

The vote to replace Mr Eido in mainly Sunni West Beirut was won easily by pro-government candidate Mohammad Amin Itani, as expected.

Intense rivalry

The headline contest, however, was in the deeply divided Maronite Christian heartland of Metn.

Officials said Camille Khoury, of Mr Aoun's Free Patriotic Movement, had won by just over 400 votes, or about 0.5% of the vote.

He defeated Amin Gemayel, a former president, leader of the Phalange Party, and father of the assassinated MP whose seat was up for grabs.

Before the result was announced, Mr Gemayel alleged voting irregularities and demanded a re-run in one district.

The contest reflects the bitter struggle between the Western-backed government and the opposition alliance, which includes both Mr Aoun and Hezbollah, a Shia Muslim group backed by Syria.

Mr Aoun and Mr Gemayel are seen as frontrunners in next month's race to succeed pro-Syrian President Emile Lahoud.

Under Lebanon's sectarian political system, the presidency is reserved for a Maronite Christian, and is elected by parliament.

The rivalry between the two political veterans has intensified amid a wider conflict between Lebanon's pro- and anti-Syrian groups.

Call for calm

Mr Gemayel and his allies accuse Syria of orchestrating the shooting of his son last November and other anti-Syrian figures including Mr Eido.

Mr Aoun is a former military leader who was once a vocal critic of Syria and won a vast majority of the Christian vote in polls in 2005.

But support for the FPM slipped after Mr Aoun unexpectedly allied himself to the pro-Syria opposition movement Hezbollah.

Turnout was high in Sunday's vote.

There were clashes between the rival Christian factions in the run-up to the vote, and reports of some fights after polls closed being broken up by police.

Both leaders called for calm.

"We hope that everything goes quietly tonight," said Mr Aoun.

Deep problems

Prime Minister Fouad Siniora said the largely orderly vote had been a civilized response to political assassination.

Pierre Gemayel and Walid Eido were the latest in a growing line of prominent anti-Syrians to be killed on the streets of Beirut.

The assassination of former Lebanese Prime Minister Rafik Hariri in 2005 drew widespread international condemnation, after which Syrian troops withdrew, after many years in Lebanon.

Syria has been accused of involvement in the attack - a claim it rejects.

Lebanon is a divided country facing its biggest political crisis in years, and these by-election results will not solve the country's deeper problems, says the BBC's Christian Fraser in Beirut.

In fact they may only raise more questions, he says.

President Lahoud, who is allied with the Hezbollah-led opposition, has not given the polls his blessing, and parliamentary speaker Nabih Berri has said he will not recognize the results.

PHOTO CAPTION

Amin Gemayel and Michel Aoun