A small protest in Libya for the release of a human-rights lawyer flared into an antiregime demonstration that was violently ended by police and government supporters, according to local media reports and a human-rights group monitoring the event.

Activists, using social-networking sites, have called for more protests Thursday against the regime of longtime leader Col. Moammar Gadhafi, hoping to gain momentum from revolts in Tunisia, Egypt and elsewhere in the Mideast.

Mr....