The Tunisian government has banned a Salafist group from holding its annual congress, according to the interior ministry.
The ministry said on Friday that Ansar al-Sharia's congress planned for Saturday posed a threat to public order.
"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," the ministry announced.
"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 promised a tough response to "anyone who tries to attack the forces of order" and said the police and army were on "high alert to protect the security of citizens and their property".
Al Jazeera's Youssef Gaigi, reporting from the capital, Tunis, said there was a large presence of security forces in the city and on the road to Kairouan, the venue for the meeting, and that people suspected of heading there were being intercepted.
Earlier, Lotfi Ben Jeddou, the interior minister, said Ansar al-Sharia, which does not recognise the authority of the state, had not submitted a request for authorisation to hold the meeting.
Rached Ghannouchi, who heads the moderate Islamist ruling party Ennahda, said this week that the government would not allow Ansar al-Sharia to hold its congress, in advance of the final ruling.
Ansar al-Sharia's remained defiant, pledging to go ahead with the gathering in the historical central city of Kairouan and warned that the government will 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," Seifeddine Rais, the group's spokesman said, said on Thursday.
Rais said more than 40,000 people were expected to attend the congress.
US embassy warning
Ansar al-Sharia urged its supporters on Friday evening to travel in groups, without calling off the event.
"We advise our brothers coming to Kairouan to travel in groups and not to be separated because the agents of the tyrant are blocking most intersections and provoking our brothers by showing their weapons," it said on its Facebook page.
Salafists advocate an ultra-conservative brand of Sunni Islam, and Ansar al-Sharia is considered the most radical of the groups that emerged after the 2011 revolution that toppled the longtime ruler Zine El Abidine Ben Ali.
Islamists have been blamed for a wave of violence across the country, including an attack on the US embassy in September that left four assailants dead.
The group's fugitive leader, Saif Allah Bin Hussein, a former al-Qaeda fighter in Afghanistan, gave warning last week he would wage war against the government, accusing it of pursuing policies against Islam.
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 fighters being hunted by the army in the border region with Algeria.