The Tunisian government has definitively banned hardline Salafist group Ansar al-Sharia from holding its annual congress at the weekend, the interior ministry announced on Friday.
"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.
Angered by that, the group vowed to go ahead with the gathering in the historical central city of Kairouan and warned that the government would be responsible for any violence.
"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 on Thursday.
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.
The ministry statement warned that "all those who defy the authority of the state and its institutions, who try to sow chaos, who incite violence and hatred will bear all the responsibility".
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."
Earlier, Ben Jeddou warned that the government would not tolerate unrest.
"We have special forces to protect Tunisia," he said.
"We do not accept death threats or incitement to hatred. We do not accept to be treated as tyrants."
Ansar al-Sharia did not apply for permission to hold the congress, he said, urging the Salafists to embrace "wisdom".
"We told them there should be no violence, physical or verbal, and that they should limit themselves to preaching (Islam). But so far they did not request a permit."
"God willing," Ben Jeddou said, "we will not have to resort to violence" if the Salafists break the law.
"We don't want a confrontation with them. They are Tunisians. We did not close their mosques; we did not prevent them from preaching. They are they ones who are raising the stakes."
A US embassy travel advisory warned its citizens against travelling to Kairouan at the weekend, saying "large rallies and demonstrations are possible" if Ansar al-Sharia's congress goes ahead.
"There is the potential for disruption to traffic in the area of Kairouan and possible confrontations with security forces. The embassy recommends against all travel to Kairouan during this period."
Salafists advocate an ultra-conservative brand of Sunni Islam, and Ansar al-Sharia is considered the most radical of the extremist groups that emerged in Tunisia after the 2011 revolution that overthrew veteran strongman Zine El Abidine Ben Ali.
Islamists have been blamed for a wave of violence across the country, including an attack on the American embassy in September that left four assailants dead.
The group's fugitive leader, Saif Allah Bin Hussein, a former Al-Qaeda fighter in Afghanistan, warned last week he would wage war against the government, accusing it of policies in breach of Islam.
Ennahda leader Ghannouchi has said the Salafists were behind the brutal killing of a policeman this month, and that they acted in response to a fatwa, or religious decree, issued by a top cleric.
The victim was slaughtered, stripped and hidden in a mosque.
Bin Hussein, who goes by the name of Abu Iyadh, was jailed under Ben Ali but freed after the uprising.
His movement has denied any connection with jihadists being hunted by the army in the border region with Algeria.