BEIRUT, Lebanon – A top Shiite cleric warned Monday that violence in Beirut could spin out of control, a day after seven protesters died in rioting and clashes in the city’s southern suburbs that were reminiscent of Lebanon’s 15-year civil war.

But the area was calm Monday as troops patrolled and Shiite Muslims buried their dead.

Prime Minister Fuad Saniora declared a national day of mourning and ordered universities and schools closed to prevent further friction among political factions.

Sunday’s death toll was the highest for a street disturbance since the country plunged into a crisis three years ago with the assassination of former Prime Minister Rafik Hariri, a turning point in Lebanese politics that sparked local and international outrage and forced the Syrian army to withdraw after 30 years of control.

What started as an angry protest by government opponents against electricity rationing degenerated into street violence and clashes with troops. The violence was confined to mainly Shiite areas, and the army deployed in what appeared to be an attempt to prevent the unrest from spreading to nearby Sunni Muslim and Christian areas.

The army, seeking to ease tensions with the Shiite community that has long been a strong ally, said Monday it regretted the loss of life and pledged “extreme seriousness” in an investigation into what happened.

The violence “only serves the enemies of the nation, first and foremost the Israeli enemy,” said a military statement, issued after the army chief met the parliament speaker and Amal opposition leader, Nabih Berri.

Lebanon’s top Shiite Muslim cleric joined the main Shiite groups, Hezbollah and Amal, in calling for an investigation into the “cold-blooded killings” and warned that matters could get out of control.

The army “must clarify as soon as possible” the events of Sunday, “so that matters will not aggravate as a result of the state of political turmoil and public discontent,” Grand Ayatollah Mohammed Hussein Fadlallah of Lebanon’s 1.2 million Shiites said in a statement.

The fighting ignited memories of the 1975-90 civil war and came during a political fight over who will become Lebanon’s next president. The clashes erupted along the war’s former demarcation line between Christian and Muslim areas and near a district where the bloody conflict, which killed 150,000, began.

A hand grenade tossed by rioters into that district, Ein el-Rummaneh, injured four people, and an opposition TV station claimed some shooting may have come from the opposing Christian side, and not only from the army.

“Our nation is going through its most difficult and dangerous times and circumstances, threatening the collapse in our hands of what we have built in the past years,” Saniora said in a statement late Sunday.

He urged people to wait for an investigation into how the shootings occurred and to rally behind the army, declaring the victims as “martyrs of all the nation.”

One of the people killed was a local official with the Amal opposition group, Ahmed Hamzeh, who had been working with the army to reduce tensions, security officials and the party said. Another was a paramedic of a Muslim ambulance service affiliated with Hezbollah.

Lebanon is embroiled in its worst political crisis since the civil war. Former President Emile Lahoud left office on Nov. 23 without a successor, and parliament has so far failed to elect the army chief to replace him amid bickering between the parliament majority and the opposition.

The Shiites’ five ministers bolted out of Cabinet 14 months ago and have not been replaced. The pro-Western parliament majority has claimed that south Beirut suburbs harbor bombers such as those who struck most recently on Friday, killing a senior pro-majority anti-terror police official.

The pro-government coalition, which is backed by the United States and pro-American Arab states, blamed the violence on Syria and Iran, who back the opposition led mainly by the Shiite militant Hezbollah and Amal. But Hezbollah blamed the government for the trouble.