One police officer died after he tackled looters amid the disturbances Wednesday, Tunisia's state-run news agency TAP quoted an interior ministry official as saying.
Protesters also rallied elsewhere Wednesday, including the central town of Sidi Bouzid, the birthplace of the revolution that toppled former President Zine al-Abidine Ben Ali two years ago.
The French Embassy in Tunisia said all French-run schools in the North African country would be closed on Friday and Saturday.
Belaid's family told local media his funeral would be held Friday, according to TAP.
Belaid routinely received death threats for his outspoken criticism of Tunisia's moderate Islamist-led government. He talked about the bullying on his frequent television appearances but said he didn't fear for his life.
Belaid had criticized the government, saying it was not doing enough to take on hardline Salafists.
No one has claimed responsibility for the attack. Some speculate an extremist Salafist group may have been behind it.
Support for Belaid went beyond his own party, the secular-leftist Democratic Patriots. He was the voice of a large coalition of secular opposition parties known as the Popular Front and had a reputation for decrying violence.
Amna Guellali, of the rights group Human Rights Watch, said the government bears some responsibility for his death 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.