Gunfights have broken out in several parts of Lebanon following Sunday's funeral of Wissam al-Hassan.
Hassan was the head of the intelligence wing of the Internal Security Forces and an outspoken critic of Syrian President Bashar al-Assad.
He was killed in a car bomb blast along with his bodyguard and a bystander on Friday.
Violence overnight left at least three people dead in the northern city of Tripoli. Soldiers clashed with unidentified armed men in the capital, Beirut.
The army said that "it is willing to act very firmly and concisely to confront this phase. Since Lebanon is going thorugh a very critical time and that tensions have reached unprecedented levels."