Supporters of Lebanon's caretaker Prime Minister Saad Hariri have taken to the streets in protest at efforts by Hezbollah to form the next government.
Protesters, who accuse the Shia Islamist movement of staging a coup, are blocking roads and burning tyres in several towns and cities.
Hezbollah and its allies earlier won the nomination of their candidate Najib Mikati as the next prime minister.
The US expressed "great concern" over Hezbollah dominating government.
"Our view of Hezbollah is very well-known. We see it as a terrorist organisation, and would have great concerns about a government within which Hezbollah plays a leading role," State Department spokesman PJ Crowley said.
"The larger the role played by Hezbollah in this government the more problematic our relationship will be."
Lebanon's national unity cabinet collapsed on 12 January after a row over a UN tribunal investigating the 2005 murder of Mr Hariri's father, Rafik.
Saad Hariri had refused to renounce the UN inquiry that correspondents say will blame senior Hezbollah figures for the murder. Hezbollah says the investigation is politically motivated.
Reports on Monday said demonstrations broke out in various regions with a significant Sunni Muslim population.
In the northern port city of Tripoli - Mr Mikati's home town and Lebanon's main Sunni bastion - protesters chanted "Sunni blood is boiling" and "Hezbollah, party of the devil".
"Saad Hariri is the only man who represents the Sunni faith," Sheikh Arslan Malas, a local cleric, told the crowd. "We will not accept (Hezbollah leader) Hassan Nasrallah choosing our prime minister."
Tripoli MP Mohammed Kabbara called for a "day of anger" on Tuesday, saying: "This aggression against the Sunni confession and the nation is unacceptable."
The BBC's Kevin Connolly says protesters also symbolically blocked the main road to Syria - the regional power broker which is one of Hezbollah's main backers.
Under Lebanon's power-sharing system, the post of prime minister is reserved for a Sunni, while the president must be a Maronite Christian and the speaker of parliament a Shia.
During consultations with President Michel Suleiman at the presidential palace on Monday Mr Mikati - a Sunni and US-educated billionaire businessman - won the support of 65 of the 128 members of the Chamber of Deputies.
It was the decision of Druze leader Walid Jumblatt and six members of his Progressive Socialist Party to switch their allegiance from Mr Hariri that swung the vote.
President Suleiman is due to announce his choice for the post on Tuesday after meeting all groups in parliament.
However, Mr Hariri has already said he will refuse to join a Hezbollah-led coalition government. Hezbollah is backed by Iran.
Hezbollah leader Sheikh Hassan Nasrallah said that if the group's candidate was appointed prime minister, it would try to form another national unity government that included Mr Hariri's Western-backed Future Movement.
Hezbollah is on the official US list of foreign terrorist organisations and is subject to financial and travel sanctions.