Lebanon's army came in for rare criticism yesterday from the country's powerful Shia Hizbollah movement, a day after at least seven people died in riots that started as a protest against power cuts.

The deaths in some of the worst civil unrest in the country since the 1975-1990 civil war heightened political tensions that are running high over disagreements on how to fill the vacant presidency. They may deal a fatal blow to the chances of Michel Suleiman, an army commander, becoming the next head of state.

Hizbollah, backed by Syria and Iran, and the de facto leader of the opposition against the western-backed government, has demanded an inquiry into the killing of the protesters, almost all opposition members and Shia.

The movement's al-Manar television network said: "Soldiers conducted several measures which led to terrifying some residents of Beirut's southern suburb."

The riots started on Sunday afternoon when residents of the Shiyah district in south Beirut took to the streets, burning tyres and throwing stones at the army. The clashes spread to other parts of the city and the country after the death of the first protester, a member of the Shia Amal movement, which is a Hizbollah ally.

General Suleiman met Nabih Berri, leader of the Amal movement, yesterday and the two agreed the army would conduct a "swift and serious" investigation.

The government declared a day of mourning yesterday, shutting down universities and schools. The army maintained a high state of alert in the capital as Shia mourners buried their dead.

The ruling anti-Syrian March 14 bloc has blamed the opposition for encouraging clashes and riots with economic pretexts over the last couple of weeks as a way of increasing pressure on the government and the army. The bloc's leaders also accuse Syria of assassinations in Lebanon since 2005.

The attacks have escalated during the presidential vacuum that started at the end of November, with three bombings in the past two months. The army was targeted for the first time in December with the killing of its chief of operations.

In the southern Shia neighbourhoods of Beirut, people accused the army and pro-government Christian militias of being responsible for Sunday's killings.

Criticism of the army is rare in Lebanon, especially since the military took on Islamic militants in the Palestinian refugee camp Nahr el-Bared last year.