Lebanon's army said on Monday the nation faced a critical time after violent demonstrations against the killing of a senior intelligence officer, saying it would act decisively to curb tension and calling for calm from politicians and protesters.

A day after angry mourners tried to storm Prime Minister Najib Mikati's offices in central Beirut, the army also warned that targeting public or private institutions crossed a "red line", and it would respond.

"The last few hours have proven without a doubt that the country is going through a decisive and critical time and the level of tension in some regions is rising to unprecedented levels," a statement issued by the army leadership said.

"We call on all political leaders to be cautious when expressing their stances and opinions... because the fate of the country is on the line", the strongly worded statement said.

Gunmen have taken to the streets in Beirut and other towns, erecting barriers and burning tyres to block major roads. At least four people have been killed in clashes in the northern city of Tripoli and five people were wounded in Beirut on Monday.

While pledging to leave politicians to find a political solution to the crisis triggered by Friday's assassination of Brigadier General Wissam al-Hassan, the army also pledged "to stamp out any security violation and safeguard national peace".

"The army leadership reiterates, by action not words, that security is a red line, and likewise the targeting of official institutions and violation of private and public property," it said.

It called on people from across Lebanon's diverse sectarian and political affiliations "to exercise the highest level of national responsibility at this testing time", and end protests which have blocked roads across the country since Friday.

The army "will take decisive measures, especially in areas with rising religious and sectarian tensions, to prevent Lebanon being transformed again into a place for regional settling of scores, and to prevent the exploitation of the assassination of the martyr Wissam Hassan to transform it into an opportunity to assassinate a whole country."-Reuters