The United States is to help Lebanon investigate a car bomb that killed the country's domestic intelligence chief on Friday in Beirut.
Police fired bullets and tear gas on Sunday after protesters tried to storm the offices of Prime Minister Najib Mikati, demanding that he quit over the assassination of General Wissam al-Hassan.
The State Department said Washington is to help investigate the bombing.
The opposition and its supporters believe Mr Mikati is too close to President Bashar al-Assad of Syria and Hizbollah, which is part of Mr Mikati's government.
Two former prime ministers - Saad Hariri and Fouad Siniora - had to intervene to urge their supporters to remain calm.
General Hassan, 47, was close to the opposition and the Hariri family. His investigation into the assassination of former Prime Minister Rafik Hariri in 2005, implicated Damascus.
Mr Hassan also recently organised the arrest of a former minister accused of planning a bombing campaign in Lebanon.
General Hassan is seen as a martyr who was killed trying to protect his country. He was buried on Sunday alongside former Prime Minister Hariri.