Protests also have erupted in the central towns of Gafsa and Sidi Bouzid, the birthplace of the revolution.

Belaid, whose party was the secular-leftist Democratic Patriots, also was the voice of a coalition of secular opposition parties known as the Popular Front.

He routinely received death threats for his criticism of Tunisia's moderate Islamist-led government. He talked about the threats on his frequent television appearances, but said he didn't fear for his life.

Official investigators have yet to reach a conclusion on who may have been responsible.

Amna Guellali of Human Rights Watch said the government bears some responsibility because of its "laxity" in failing to respond to a climate of rising political violence.

"We warned the government that these incidents of violence should be investigated thoroughly and that people who have perpetrated these acts should be punished ... but we haven't heard anything back," she told CNN in Tunis.

Guellali cited calls by preachers in some mosques in July for the killing of certain Tunisian political figures and personalities, including Belaid.

"We didn't see the government reacting to these calls of clear incitement to murder," she said. "A government has to protect its citizens ... especially if there are clear threats against this person."