Argentine President Cristina Fernandez de Kirchner said she is convinced the death of top prosecutor Alberto Nisman was not suicide.

Nisman, who accused Kirchner of whitewashing the investigation of Iran over the 1994 bombing of AMIA Jewish community centre in Buenos Aires, was found shot dead in his Puerto Madero apartment on Sunday (18 January). A 22 calibre pistol was by his side.

Prosecutors investigating his death said it appeared to be suicide.

But in a letter published on Thursday, Kirchner said via her website she was convinced he was killed.

He died just hours before he was due to give evidence about the allegations in a closed-door hearing to Congress.

The 1994 bombing killed 85 people and was dubbed the worst terrorist attack in Argentina.

Nisman alleged the government plotted to clear the Iranian suspects involved in the bombing in exchange for arranging an oil/grain trade deal with Iran.

He had accused Kirchner of trying to derail his investigation into the bombing, who turned to Facebook to defend herself.

She said the two witnesses in his case were falsely presented as state intelligence agents.

"Nisman's accusation not only collapses, but becomes a real political and legal scandal ... That's the key. Prosecutor Nisman did not know that the men identified as intelligence agents were in fact not," she said.

Hundreds of people have since taken to the streets of Buenos Aires demanding justice for Nisman's death.