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Security forces and militiamen backed by helicopters and warplanes besieged parts of Tripoli overnight, according to witnesses and news reports. Fighting was heavy at times, and the streets were thick with special forces loyal to Colonel Qaddafi fighting alongside mercenaries. Roving the streets in trucks, they shot freely as planes dropped what witnesses described as “small bombs” and helicopters fired on protesters.
Hundreds of Qaddafi supporters took over the central Green Square in the capital after truckloads of militiamen arrived and opened fire on protesters late Monday night, scattering them. Residents said they now feared to leave their houses.

NY Times: Qaddafi’s Grip Falters as His Forces Take On Protesters, February 22, 2011

There were also reports of bystanders, including women and children, leaping to their deaths from high bridges as they tried to escape battle-hardened mercenaries from neighbouring countries like Chad.
Meanwhile, the government in Tripoli has shut down a range of media, including internet providers, social networking sites and the signals of western news channels. 

The Telegraph: Libya protests: 'foreign mercenaries using heavy weapons against demonstrators, February 18, 2011

The Globe and Mail: Gadhafi's influence on Africa (graphic)

These are two severely upsetting videos from Bahrain where the military shot peaceful protesters last week.