Schools and universities closed and troops patrolled the streets of Beirut to prevent fresh outbreaks of rioting.

“This is an hour of sadness. Our country is passing through the most dangerous times,” said Fouad Siniora, the Prime Minister.

Lebanese newspapers said that the riot evoked grim images of the 1975-90 civil war. “Black Sunday”, the pro-government Al-Mustaqbal daily labelled it. A headline in the independent French-language L'Orient-Le Jour said: “The demons of discord are attempting to reignite the fires of civil war”

The demonstrations began late yesterday afternoon when supporters of the militant Hezbollah organisation and the allied Amal movement used burning tyres to block streets in the Shiyah neighbourhood of southern Beirut to protest against recent power cuts.

The mainly Shia district has witnessed several small demonstrations over the past two weeks, but all ended peacefully after the arrival of Lebanese troops. Yesterday's demonstration, bigger than previous protests, turned violent when an Amal official was shot dead by an unknown gunman.

The protests spread to other areas of Beirut as club-wielding youths smashed shop windows and damaged parked cars. Gunmen were seen roaming the streets and taking up position on roof tops.

Lebanese troops flooded the area to prevent the demonstrators reaching adjacent Christian and Sunni neighbourhoods. The army restored order late at night after another seven people were shot dead, among them four Hezbollah activists, a rescue worker and a civilian. Bulldozers re-opened roads after late-night rainfall helped to douse the flames from burning barricades.

Southern Beirut was quiet this morning, but tensions were running high. “They killed my friend, Mahmoud Hayek, who lived on the other side of the street from me,” said Ali Hamieh, a resident of Shiyah. “They also killed a Hezbollah bodyguard. There's a lot of wounded in the hospitals.”

Mr Hamieh said that most residents of the area believed the shooting was from members of the Lebanese Forces, a pro-government Christian party that fielded a powerful militia during the civil war years. “The snipers were in the high buildings overlooking our area and shooting at anything that moved,” he said.

Hezbollah and Amal leaders were urging calm among their supporters, but “people are angry and some want to take revenge”.

The protests came at a moment of high tension in Lebanon, with a continued political impasse and only two days after a senior police officer was killed in a car bomb attck, the latest victim in a three-year string of murders.

The political standoff pits the supporters of the Western-backed Government against the pro-Syrian Opposition, which is spearheaded by Hezbollah. The sides are gridlocked over the composition of a new coalition government and key civil service appointments. The deadlock has left Lebanon without a president since November and has defied intense European and Arab mediation.

The March 14 coalition, which supports the Government, blamed the Opposition for Sunday's violence. “The forces of the Syrian-Iranian axis are fomenting unrest and these events are very dangerous,” it said in a statement.

Hezbollah demanded an inquiry into the shootings, warning that any cover-up would be a threat “to stability and civil peace” in Lebanon.

“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?”

Last night's violence was the worst since January last year when a riot in a university cafeteria spilt into the streets, ending in the deaths of seven people and an army-imposed curfew. That incident underlined the dangerous level of tension among rival partisans and compelled the opposition to tone down a campaign of street action to topple the Government.

A year later, the political crisis in Lebanon is graver than ever, and opposition leaders have indicated that they could launch a nationwide campaign of civil disobedience. Many Lebanese fear that last night's disturbances are a foretaste of what will follow if the crisis is not resolved.