Inside the city, helicopters hovered overhead and police checkpoints were set up, while special units were deployed in the square facing the mosque which is the venue for the congress.
A local police officer, declining to be named, said: "We have taken all measures to ensure the meeting does not go ahead... We will not allow those coming for this congress to enter the city."
Meanwhile, maps were posted on Facebook pages close to the Salafist movement locating the checkpoints and possible routes to avoid them.
"The meeting will go ahead," an Ansar al-Sharia official, Sami Essid, told AFP, although the movement's Facebook page called for restraint and not to give way to police provocation.
Ansar al-Sharia has urged its supporters to travel to the venue in groups in a bid to get past police.
An AFP correspondent saw masked and armed policemen storm an electrical store in Kairouan and arrest a man holding black Ansar flags, while Tunisian media reported several arrests in towns around the country.
In Tunis, large numbers of police vans and army trucks were visible both in the city centre and in neighbourhoods regarded as Salafist strongholds.
As tensions mounted, a US embassy travel advisory warned Americans against travelling to Kairouan, saying "large rallies and demonstrations are possible" if the congress goes ahead.
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 party Ennahda, accusing it of policies in breach of Islam.
The interior ministry on Friday said Ansar al-Sharia posed a threat to public order as it confirmed the ban on the planned congress.
"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," it 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.
The interior ministry has 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".
Radical Islamist group Hizb ut Tahrir condemned the interior ministry ban but also appealed to Ansar al-Sharia to postpone the congress to avoid bloodshed.
"We say to Ansar al-Sharia that we consider it wise and a priority to announce the postponement of the congress, placing the whole responsiblity for it on the government," the group said in a statement.
Otherwise, "Sunday will be a day of bloody confrontation."
The Salafists, who advocate an ultra-conservative brand of Sunni Islam, have been blamed for a spate of attacks on police in recent months.