* Tunisia cracks down on Islamist Ansar al-Sharia group
* Suicide bomb belts found at raided house - source
* Militants use Libya chaos to get arms, training (Updates with identity of top suspect)
By Tarek Amara
TUNIS, Feb 4 (Reuters) - Tunisian police killed seven Islamist militants, including a senior commander wanted for the assassination of two opposition leaders, after a clash outside Tunis where the armed group had stashed arms and bomb belts.
The raid was one of the deadliest since Tunisian forces cracked down on the banned Islamist militant movement Ansar al-Sharia, whose leader declares allegiance to al Qaeda, and which Washington lists as a foreign terrorist group.
Gun battles broke out late on Monday night when police surrounded a house in the Raoued suburb north of Tunis, leaving one police officer and seven militants dead, the Interior Ministry said, without naming the group.
Among those killed was Kamel Ghadghadi, a senior member of the Ansar al-Sharia, wanted for killing seven soldiers, some of whom had their throats slit, and for assassinating opposition leaders Chokri Belaid and Mohamed Brahmi.
"Ghadghadi is among those killed. This is the best present for Tunisians a year after the murder of Belaid," Interior Minister Lofti Ben Jeddou told reporters at a news conference.
Officials showed reporters a photograph of what they said was Ghadghadi's corpse, wearing a suicide bomb belt. Other explosive material and weapons were also found in the house.
Raoued is a poor district close to luxury beach resorts just outside the capital. Heavily armed counter-terrorism police patrolled near the whitewashed house where the fighting took place, its outer wall pockmarked by bullet holes.
Ansar al-Sharia was one of the more radical movements to emerge after Tunisia's 2011 uprising ousted president Zine El-Abidine Ben Ali, whose autocratic regime suppressed and jailed Islamist leaders.
The rise of ultra-conservative Salafi movements who promote the establishment of an Islamic state has alarmed many in Tunisia, one of the most secular nations in the Arab world with a strong tradition of liberal education.
Ansar al-Sharia was blamed for storming the U.S. embassy in Tunis in late 2012. The moderate Islamist government at the time declared Ansar al-Sharia a terrorist organisation after accusing the group of murdering the opposition leaders.
Three years after its Arab Spring uprising, Tunisia is led by a caretaker government that took over after Islamist Ennahda party stepped down in a compromise to end a crisis sparked in part by the killing of Belaid and Brahmi.
Tunisia formally celebrates a new constitution on Friday, with French President Francois Hollande and other dignitaries invited to the ceremony to mark the North African country's progress to democracy.
The threat of Islamist militancy is among the new government's main challenges. A suicide bombing at a beach resort late last year - the first such attack in a decade - underscored Tunisia's vulnerability to jihadi violence.
Tunisian militants have used the turmoil in neighbouring Libya to get weapons and training. Some have travelled to Syria to fight for Islamist rebel groups in the civil war there. (Writing by Patrick Markey; Editing by Gareth Jones)