TRIPOLI, Libya – Libyan militiamen opened fire Friday on white-flag-waving protesters demanding their disbandment, killing at least 31 people and wounding more than 200 in a barrage of heavy machine gun and rocket-propelled grenade fire.

Libyan Prime Minister Ali Zidan blamed the protesters and the militia alike for the violence, though witnesses said they saw no protesters carrying weapons ahead of the shooting Friday afternoon. By Friday night, however, some protesters joined by other militias had armed themselves and heavy gunfire rang out in the Tripoli neighborhood where the attack happened.

Armed groups set up checkpoints across the Libyan capital, hoping to stop other militias from entering the city. Ambulance sirens wailed into the night.

The march in Tripoli by thousands of protesters was the biggest show of public anger at militias in months. Since the 2011 fall of dictator Moammar Gadhafi, hundreds of militias – many of them on government payroll – have run out of control in Libya, carving out zones of power, defying state authority and launching violent attacks.

The demonstrators waved Libyan and white flags and chanted, “We want an army, we want police,” referring to demands that the country’s weak security forces take the place of militias.

When they neared the headquarters of one of the militias, militiamen in civilian clothes and military uniforms came out of the headquarters, opening fire. Protesters ran from gunfire while carrying others covered in blood.

Libyan state television put the death toll at 31, with 235 people wounded.