Marines fired on a crowd of 1,500 protesters armed with rocks, machetes and clubs, killing 12 and wounding 23.
The confrontation that led to the shooting began when police clashed with a protester armed with a steel pipe.
The United States military has tightened security around its civilian installations after an attack on an American housing compound by a group of young protesters armed with firebombs and metal bars.
For example, concert-goers were often greeted by protesters armed with bullhorns and distribution of Gospel tracts.
Due to the number, frequency, diverseness, and exposure of the threats made by radical protesters, the police were concerned about facing protesters armed with unknown weapons and unknown intentions.
Pro-Gaddafi militia and foreign mercenaries also attacked an airport outside Misrata, which was defended by protesters armed with rifles, in what would become the Battle of Misrata.
Some of the 4,000 troops were beaten by protesters armed with bamboo sticks.
Clashes then broke out between the groups after pro-Saleh protesters, armed with knives and sticks, forced about 300 anti-government protesters to end their protests.
Pro-government protesters, armed with daggers and batons, clashed with students resulting in five injuries, before police intervened.