* Government bans radical Islamist group from gathering
* Ansar al-Sharia vows to convene "conference" anyway
* U.S. embassy warns citizens to avoid city of Kairouan
Tunisia's government on Friday barred the radical Islamist group Ansar al-Sharia from staging a weekend rally in the central city of Kairouan, saying the organisation, which openly supports al Qaeda, posed a "threat to public security."
But the group, under fugitive leader Saif-Allah Benahssine, has said it would defy the order and go ahead with its annual gathering on Sunday, which it expects to attract some 40,000 supporters.
The standoff comes as the army pursues dozens of suspected al Qaeda-linked militants near the western border with Algeria.
It also underlines growing concern in what has long been one of the most secular countries in the Arab world over the threat to stability posed by Islamist militants.
"The Interior Ministry has decided to ban the gathering of Ansar al-Sharia, which has shown distain for state institutions, incited violence against them and poses a threat to public security," the ministry said in a statement.
Ansar al-Sharia is considered the most radical of the hardline Islamist groups to emerge in Tunisia since a 2011 revolution overthrew secular dictator Zine al-Abidine Ben Ali.
It said that it did not need government authorisation to meet.
"The government will be responsible for every drop of blood spilled in Kairouan ... We will hold the conference in Kairouan, whatever the price," said Sai Eddine Rais, spokesman for the group.
Police cordoned off the roads leading into Kairouan on Friday and prevented hundreds of people from entering the city to join the gathering, while the U.S. Embassy in Tunis called on its citizens in Tunisia not to travel there.
The interior ministry said it had arrested a "religious militant" after finding weapons and bomb-making instructions at his home in the city.
Benahssine, also known as Abu Iyadh, is a former al Qaeda fighter in Afghanistan, and is wanted by police for allegedly inciting an attack on the U.S. embassy in September.
Four people were killed in the disturbances, which began as a protest over a film that mocked the Prophet Mohammed.
Earlier this year he threatened to topple Tunisia's government, led by the moderate Islamist Ennahda party, and vowed to throw Prime Minister Ali Larayedh into the "dustbin of history".
Hardline Islamist Salafists are seeking a broader role for religion in Tunisia, alarming the secular elite which fears their true agenda is to impose strict views on people and compromise individual freedom, women's rights and democracy.
Tunisian police blamed a Salafist for the assassination of secular opposition politician Chokri Belaid on Feb. 6, which provoked the biggest street protests in Tunisia since the overthrow of Ben Ali.