The police stood by and watched, the witness said, as the militiamen, still shooting, chased after the protesters. The death toll could not be determined.

The escalation of the conflict came after six days of revolt that began in Libya’s second-largest city, Benghazi, where hundreds of people were killed in clashes with security forces, according to witnesses. Human rights activists outside the country said they had confirmed more than 220 deaths. The rebellion is the latest and bloodiest so far of the uprisings that have swept across the Arab world with surprising speed in recent weeks, toppling autocrats in Egypt and Tunisia, and challenging others in Bahrain and Yemen.

The day had begun with growing signs that Colonel Qaddafi’s grip on power might be slipping, with protesters in control of Libya’s second-largest city, his security forces pulled back to key locations in the capital as government buildings smoldered, and a growing number of officials and military personnel defecting to join the revolt.

But the violence Colonel Qaddafi unleashed Monday afternoon on Tripoli demonstrated that he was willing to shed far more blood than the deposed rulers of either neighboring Egypt or Tunisia in his effort to hold on to power.