Hezbollah, Lebanon's most powerful faction, on Monday demanded to know who was behind shootings that killed seven opposition supporters in some of Beirut's worst street violence since the 1975-90 civil war.

Hezbollah, a pro-Syrian political party, said any cover up would be a threat "to stability and civil peace" in Lebanon.

The Shi'ite Muslim group, which has a guerrilla army, has been leading the opposition campaign against Prime Minister Fouad Siniora's U.S.-backed government for more than a year.

Security sources said at least 29 people were wounded in the violence on Sunday after the army moved to break up a protest by anti-government activists against power cuts.

An eighth person was killed in south Lebanon when hit by a car during a protest against the shootings, the sources said.

The army, which fired into the air to disperse the protest, has launched an investigation into who was behind the killings.

"Did those who fell as martyrs and were wounded fall by the army's bullets, and if so, who issued the order for the soldiers to fire?" Hezbollah asked in a statement. "Or was there another party, and who was it?"

The group said it held the authorities responsible for "every drop of blood spilt".

Schools and universities were closed to mark a national day of mourning called by the government. Funeral arrangements have yet to be announced.

The violence close to both Shi'ite and Christian areas was the worst since supporters of the anti-Syrian governing coalition and its pro-Damascus rivals clashed a year ago.

The crisis has exacerbated tensions between Sunni and Shi'ite Muslims and Christians allied to the rival sides.

The army has stayed neutral in the conflict and is one of the few state bodies that has functioned effectively while other institutions have been paralysed.

"Who is embroiling the army in a blood game?" asked pro-opposition newspaper al-Akhbar.

"Bloody Sunday ignites the Dahiyeh (suburb) in confrontation with the army," said the pro-government al-Nahar, in reference to the Beirut district where the violence took place.

Lebanon's crisis has left the country without a president since November.

Although army chief General Michel Suleiman has been agreed by rival leaders as the candidate to fill the post, his election has been delayed by a dispute between the sides over the make-up of a new government.