Libyan officials said 32 people had been killed and almost 400 wounded in clashes in Tripoli between militiamen and protesters.
The violence highlights the government’s inability to control the armed groups that helped overthrow Muammar Gaddafi two years ago, but kept their guns.
Demonstrators said that members of the Misrata militia had opened fire on them, as they marched towards their headquarters to demand they leave the city.
Machine guns and rocket-propelled grenades were reportedly fired into the crowd.
The protesters fled when the shots were fired, but some returned heavily armed.
The Libyan authorities are struggling to contain numerous militias who control many parts of the country.
Friday’s anti-militia protest came amid increasing calls for the armed groups to be disbanded or join the army, in line with an end of year deadline set by the government.