"President Moncef Marzouki received this morning the judge of the Tunis first instance tribunal who heard his testimony as a witness in the assassination of Chokri Belaid," said a brief statement.

The opposition leader was gunned down outside his Tunis home on February 6, with the broad daylight killing sparking clashes between protesters and police and prompting the largest anti-government demonstrations since the revolution.

His family says the president knew of death threats against Belaid, who was a vocal critic of the ruling Islamist party Ennahda, before he was murdered, claims presidential spokesman Adnene Mancer has denied.

On Thursday, Belaid's brother Abdelmajid repeated the claims to AFP.

"My brother had told me, and I quote him: 'The presidency has contacted me to tell me that I am threatened with death. And the president offered me protection but I refused because it would be tantamount to controlling me.'"

Abdelmajid added: "As far as I am concerned, the president knows who ordered the murder, those who decided to kill Chokri."

Belaid's relatives have dismissed the Tunisian government's assertion that radical Salafist Muslims carried out Belaid's murder, insisting instead that Ennahda was responsible, a claim the party strongly denies.

The killing has exposed the widening fissures between Tunisian Islamists and liberals, in the once proudly secular nation.

Interior Minister Ali Larayedh, who is also prime-minister designate, said on Tuesday that the killer, who remains at large, had been identified and that four suspected accomplices had been arrested, without giving their names.

On the same day, Abdelmajid Belaid told AFP: "It is Ennahda that gave the green light to kill my brother."

A judicial source with access to the murder file told AFP the suspected assassin was Kamel Gathgathi, from the northwestern Jendouba region, who had lived in America.

Speaking from Jendouba, Gathgathi's father Taieb confirmed that his son had studied in America and now lived in Tunis, while insisting he was innocent.

"From what I know of my son, I can say he is incapable of hurting a fly," he told private radio station Mosaique FM.

"He is a good type, an angel on earth, but if he has committed a crime then I pray to God that justice is done."

Taieb Gathgathi said his son's studies in America had been paid for by the Tunisian state and that he had subsequently been offered a job at Tunisia's interior ministry, without giving further details.

"I have seen him rarely since I separated from his mother. I've seen him maybe once since he's been in Tunis," Gathgathi added.

Belaid's murder caused the north African country's worst political crisis since the revolution two years ago that ousted strongman Zine El Abidine Ben Ali.

It prompted Hamadi Jebali to resign as prime minister last week, after his own party Ennahda dismissed his proposal to form a government of technocrats in a bid to defuse tensions.

Larayedh, also an Ennahda member, was named on Friday to replace Jebali. He has until March 8 to form a new government, which he has vowed will be "for all Tunisians".