"We have decided to prohibit this gathering, which would be in violation of the law and because of the threat it represents to public order," a statement said.
Earlier, Interior Minister Lotfi Ben Jeddou said Ansar al-Sharia, which does not recognise the authority of the state, had not submitted a request for authorisation to hold the meeting, planned for Sunday.
Rached Ghannouchi, who heads the moderate Islamist ruling party Ennahda, said this week the government had banned Ansar al-Sharia from holding its congress.
"We are not asking permission from the government to preach the word of God and we warn against any police intervention to prevent the congress from taking place," spokesman Seifeddine Rais said yesterday.
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Rais said more than 40,000 people were expected to attend the congress and warned that "the government will be responsible for any drop of blood spilt".
Prime Minister "Ali Larayedh will answer for his policies before God," Rais said.
It also warned of a harsh response to "anyone who tries to attack the forces of order" and said the police and army are on "high alert to protect the security of citizens and their property".
And it said the state is committed to "respecting the right to demonstrate peacefully, to safeguard freedom of expression, religious practice and peaceful preaching for all citizens in conformity with the laws in force.