Iranian-supporting Hezbollah's choice for Lebanon's next prime minister has won parliamentary support, capping the group's steady rise in power and sparking street riots by opponents.

Billionaire businessman and former premier Najib Mikati won a majority in two days of voting, defeating Western-backed Prime Minister Saad Hariri.

The president will now ask Mr Mikati to try to form a new government that could be controlled by Hezbollah and its allies and give the group an unprecedented level of political power in Lebanon.

Hezbollah's has grown over the past few decades from a resistance group fighting Israel to Lebanon's most powerful military and political force.

The events of the past few days drew warnings from the US that its support for Lebanon could be in jeopardy, demonstrating the risks of international isolation if Hezbollah pushes its power too far.

Hezbollah's Sunni rivals, who support Mr Hariri, demonstrated for a second day across the country including the capital Beirut and the main highway linking the capital with the southern port city of Sidon.

The largest gathering was in the northern city of Tripoli, a predominantly Sunni area and a hotbed of fundamentalists where thousands of people converged at a major square and set fire to an Al-Jazeera TV van.

Mr Hariri thanked people for their support but called for restraint.

"I understand your emotions, but this rage should not lead us to what is against our morals, faith and beliefs," he said.

Many fear Lebanon's political crisis could re-ignite sectarian fighting similar to Shiite-Sunni street clashes that killed 81 people in Beirut in 2008.