Lebanese army personnel and riot police used armoured vehicles and tear gas to keep control of the nation's capital on Tuesday, as Sunni demonstrators, angered by the rising power of Hezbollah, protested on the streets of Beirut.
On Tuesday, Lebanon's president formally appointed a Hezbollah-backed candidate as prime minister-designate and asked him to form a new government.
Billionaire businessman and former premier Najib Mikati won a majority of parliament support in two days of voting, defeating Western-backed Prime Minister Saad Hariri as the candidate for the next prime minister.
Hezbollah's Sunni rivals held protests in different parts of Lebanon, including Tripoli, the capital Beirut and the main highway linking the capital with the southern port city of Sidon.
Some protesters in Beirut gathered at the grave of former prime minister Rafik Hariri, father of US-backed caretaker PM Saad Hariri.
Hezbollah brought down Hariri's Western-backed government on January 12 when he refused the group's demand to cease cooperation with a UN-backed tribunal investigating the 2005 assassination of his father.
Members of Hezbollah, which denies any role in the killing, are widely expected to be indicted.
The largest gathering was in the northern city of Tripoli, a predominantly Sunni area, where thousands of people converged at a major square and protested by burning tires and torching a van belonging to Al-Jazeera.
A senior military official said several armed men fired in the air in west Beirut, but the army intervened and dispersed them.
Soldiers also clashed with demonstrators in the town of Naameh, south of Beirut, and two civilians were wounded, security officials said.
The protests, which were continued after nightfall, were mostly localised and not hugely disruptive.
The vote caps Hezbollah's steady rise over the past few decades from a resistance group fighting Israel to Lebanon's most powerful military and political force.
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.
Hezbollah's rise also looked likely to also raise tensions with Israel, which borders Lebanon to the south.
Hezbollah and Israel fought a short but devastating war in 2006.
Despite opposition from the Hariri camp, Mikati is seen as a relatively neutral choice who enjoys good relations with both Syrian President Bashar Assad and with Hariri.
That puts Hariri in the awkward position of rejecting a candidate who has been an ally in the past.
Hariri's bloc has insisted it will not join a government led by a Hezbollah pick, which could mean months of political deadlock ahead in Lebanon.
According to Lebanon's power-sharing system, the prime minister must be a Sunni, the parliament speaker a Shiite and the president must be a Christian Maronite.
Each faith makes up about a third of Lebanon's population of four (m) million.
Because Mikati is a Sunni, protesters accused him of being a traitor to his sect and betraying Hariri.
After it was clear that Mikati won the support of a majority of lawmakers on Tuesday, Hariri thanked people for their support but called for restraint.
Hezbollah can now either form its own government, leaving Hariri and his allies to become the opposition, or it can try to persuade Hariri to join a
national unity government.
In a speech on Sunday night, Hezbollah leader Sheik Hassan Nasrallah said he favoured a unity government.
Hariri said on Monday he would not join a government headed by a Hezbollah-backed candidate.
Hariri's Future bloc declared a day of peaceful protests Tuesday - but called it a "day of rage" and played on the sectarian dimension of the conflict.