Protests have also erupted in the central towns of Gafsa and Sidi Bouzid, the birthplace of the revolution.
Support for Belaid went beyond his party, the secular-leftist Democratic Patriots. He was also the voice of a coalition of secular opposition parties known as the Popular Front and decried violence.
But Belaid 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 the rights group Human Rights Watch, said the government itself 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.
She cited calls by preachers in some mosques in July for the killing of certain Tunisian political figures and personalities, including Belaid, she said.
"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."