A demonstration by Hizbulah and the Amal Movement against Tarek Bitar, the judge in charge of the investigation into the explosion at the port last year, led to a shooting that has so far left six dead and some 60 wounded.
A short distance from the courthouse, hundreds of demonstrators demanded the dismissal of Bitar, whom the two main political organizations of Lebanese Shiite Muslims accuse of politicizing the process.
The army cordoned off the area and launched an operation with tanks and heavy vehicles to bring the situation under control.
Military commanders warned on social media that soldiers would open fire on any gunmen and called for the evacuation of civilians.
Prime Minister Najib Miqati called on citizens to remain calm and not to be drawn into the violence.
With the same request, Hizbulah and Amal called on their followers to avoid falling into the trap of a sectarian clash and accused armed and organized groups of attacking their supporters.
According to the two groups, these actions were aimed at covering up responsibility for the catastrophe at the port terminal, which left more than 200 dead and some 6,500 injured.
Al Manar TV channel blamed the attacks on the Christian Lebanese Forces party, whose supporters have a strong presence in the area where the events took place.