Members of Tunisia's governing Islamist party, Ennahda, have called for a rally, a day after huge crowds attended a murdered opposition leader's funeral, BBC News reported.
A post on the group's Facebook page urged people to gather at 14:00 (13:00 GMT) on Habib Bourguiba Avenue in the centre of the capital, Tunis.
The rally would defend the legitimacy of the National Constituent Assembly and fight against violence, it added.
Opposition supporters have blamed Ennahda for the death of Chokri Belaid.
Chokri Belaid's widow Basma Khalfawi said she would file a lawsuit against the party's leader, Rachid Ghannouchi.
Mr Ghannouchi has condemned the murder and denied any involvement. The government has blamed opponents of democracy.
Mr Belaid, a lawyer and secular political figure, was shot by a gunman as he left home for work on Wednesday. The previous day he had accused a faction inside Ennahda of being behind attacks on the opposition.