Tunisian Salafist movement Ansar al-Sharia called on its hardline Islamist supporters to defy a government ban on its annual congress, setting the scene for a possible showdown Saturday on the eve of the planned meeting.
Ansar al-Sharia urged its supporters to travel to the venue in the historical central city of Kairouan in groups in a bid to get past police, adding to security concerns.
"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.
With tensions rising, a US embassy travel advisory warned Americans against travelling to Kairouan, saying "large rallies and demonstrations are possible" if the 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."
The Salafists have been blamed for a wave of violence across Tunisia, including an attack on the US embassy in September that left four assailants dead.
Ansar al-Sharia is considered the most radical of the extremist groups that emerged after the 2011 revolution that ousted veteran strongman Zine El Abidine Ben Ali.
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 led by moderate Islamist group Ennahda, accusing it of policies in breach of Islam.
The interior ministry said Ansar al-Sharia posed a threat to public order as it confirmed the ban on the planned congress 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," the ministry announced.
Ahead of the ministry's announcement, Ansar al-Sharia, which does not recognise the authority of the state, warned that it would hold the government 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.
Rais said more than 40,000 people were expected to attend, adding: "The government will be responsible for any drop of blood spilt".
The interior ministry retorted 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 promised a tough 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".
Interior Minister Lotfi Ben Jeddou accused Ansar al-Islam of seeking a showdown.
"We don't want a confrontation with them. They are Tunisians," Ben Jeddou said.
"We did not close their mosques; we did not prevent them from preaching. They are they ones who are raising the stakes."
The Salafists, who advocate an ultra-conservative brand of Sunni Islam, have been blamed for a spate of attacks on police in recent months.