CAIRO  Thousands gathered Friday for a third day of violent demonstrations in Benghazi, Libya’s second-largest city, in an unprecedented challenge to the mercurial 41-year reign of Col. Muammar el-Qaddafi. Human rights groups said 24 people had been killed across the North African country, though activists say the count may be far higher.

The escalating unrest bears the hallmarks of uprisings in Egypt and Tunisia, as protesters copy slogans heard there. But as in Bahrain and Iran, the police and the army have moved quickly to crush unrest. Residents say the government has mobilized young civilian supporters in the capital and other towns and deployed foreign mercenaries in eastern Libya, long the most restive region.

Libya demonstrates both the power and the limits of the Arab uprisings. The country, though the most isolated in the region, is not disconnected enough to black out the news of autocrats falling in two of its immediate neighbors. But information about what is happening inside Libya  and the ability of protesters to mobilize world opinion on their behalf  is far more limited.

A refrain of opposition leaders was that the world was failing to act, even as they sought to post videos, statements and testimony on social networking sites with mixed success.