Tens of thousands of Salafists are expected to attend the annual congress of Ansar al-Sharia, a radical Tunisian group whose leader is on the run, one of the organisers said Wednesday.

"On Sunday, we will God willing hold our congress and there will be more than 40,000 of us in Kairouan," Sami Essid told AFP.

He said the annual gathering, just the third to be held by the radical Islamist group, will take place after afternoon prayers at the Great Mosque in Kairouan, in central Tunisia, considered Islam's fourth-holiest city.

The interior ministry, which requires political parties and associations to obtain permission for all public activity, has not yet said whether the congress will be allowed to take place.

"We are currently studying the matter before taking a decision," ministry spokesman Mohamed Ali Aroui said without elaborating.

But Essid, who is close to the hardline group's fugitive leader Abu Iyadh, insisted they did not need permission to hold their meeting.

Registered as a non-governmental organisation in April 2011, Ansar al-Sharia is the most radical of the Islamist movements that emerged in Tunisia after the revolution earlier that year that overthrew secular dictator Zine El Abidine Ben Ali.

Abu Iyadh, whose real name is Seif Allah Ibn Hussein, is accused of orchestrating numerous acts of violence and has been on the run since September after Islamist protesters attacked the US embassy in Tunis.

A veteran jihadist who fought with Al-Qaeda in Afghanistan, he threatened last weekend to wage war against the government led by moderate Islamist party Ennahda, saying that by targeting his followers it was attacking Islam.

"To the tyrants who think they are Islamists... know that the stupid things you are doing are dragging you to war," he said in a message posted online, adding that young Salafists "won't hesitate to sacrifice themselves for their religion in Kairouan."

The authorities have hardened their position towards extremists in recent months, notably by stepping up military operations against jihadists with suspected links to Al-Qaeda who are hiding in the western region of Kasserine that borders Algeria.

Interior Minister Lotfi Ben Jeddou said last week that he would bring to justice "anyone inciting to murder or hatred... or who pitches tents for preaching in," in a clear reference to the Salafists.