* Some 120 wounded, many trapped inside damaged buildings

* Not known if Hezbollah figure was target

* Blast comes amid Syria tension, after car bomb last month (Updates death toll; adds details, quote, background throughout)

By Laila Bassam

BEIRUT, Aug 15 (Reuters) - A powerful explosion struck the southern Beirut stronghold of Lebanon's militant Hezbollah group on Thursday, killing 10 people, wounding 120 and trapping many others inside damaged buildings, witnesses and emergency officials said.

The blast, a month after a car bomb wounded more than 50 people in the same district of the Lebanese capital, came amid sectarian tensions over the intervention of Shi'ite Muslim Hezbollah against Sunni rebels in Syria's civil war.

Residents of southern Beirut say Hezbollah, backed by Iran and Syria, was on high alert and stepped up security in the area after warnings from Syria rebels of possible retaliation for the group's support for President Bashar al-Assad.

It was not immediately known whether the explosion targeted any Hezbollah figure but it took place in a crowded residential area.

"I don't know what happened. It's as if we were struck by an earthquake," one young man at the scene told Reuters, bleeding from a wound to his stomach.

A spokesman for Lebanon's Red Cross said 10 people were killed and another 120 were wounded.

At the heart of the site, where fires still raged almost an hour after the blast, the twisted remains of a large van could be seen and some witnesses said they believed the explosion was caused by a car bomb.

Many cars were damaged in the explosion and the blast sent a column of black smoke over the densely populated area. The facades of several residential buildings were damaged. Charred bodies were seen inside cars caught in the explosion.

Al Mayadeen television said some people were still trapped inside apartments at the scene, close to the Sayyed al-Shuhadaa (Martyrs) complex, where Hezbollah leader Sayyed Hassan Nasrallah often addresses his followers.

Hezbollah supporters immediately put up a security cordon around the area, witnesses said.

HUGE EXPLOSION

"I heard a huge explosion. It threw me several metres," said a woman in her 50s who said she had been talking to her brother in his shop.

"I don't know what happened to my brother. I can't find him," she said, bleeding from wounds to hands and face.

Syria's conflagration has spread to neighbouring Lebanon, where there have been outbreaks of fighting reflecting the renewed sectarian tension now spreading through the Middle East.

Lebanon's Sunni Muslims mostly support the rebels in Syria, while Shi'ites have largely supported Assad, who is part of the minority Alawite sect, an offshoot of Shi'ite Islam.

Hezbollah leader Nasrallah has promised that his group will continue fighting for Assad after it spearheaded the recapture of the strategic town of Qusair in June.

There have been two attacks in southern Beirut this year. Two months before the July 9 car bomb, two rockets were fired into the area.

In October last year, a car bomb in the east of the capital killed a senior intelligence official, Wissam al-Hassan, who was close to the country's leading Sunni opposition party, which has supported the uprising in Syria. (Writing by Dominic Evans; Editing by Alison Williams)

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