President Moncef Marzouki has cancelled his participation in the Organisation of Islamic Cooperation summit in Cairo and is heading back to Tunisia after the murder of opposition leader Chokri Belaid, the presidency said on Wednesday.
"He returns early this afternoon. It has just been decided," presidential adviser Ghassen Dridi told AFP by phone.
Marzouki was in Strasbourg on Wednesday morning, where he took part in a session of the European parliament and met French President Francois Hollande, and was due to fly to Cairo on Thursday.
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The presidency denounced the killing of Belaid as an "odious" crime designed to "lead the Tunisian people to violence," calling in a statement for "restraint and wisdom."
Belaid, who headed the opposition Democratic Patriots party and was a harsh critic of Tunisia's Islamist-led government, was shot dead as he was leaving his home on Wednesday morning.
His family swiftly accused Ennahda, the Islamist party that heads Tunisia's coalition government of being behind the murder.
His assassination comes at a time when Tunisia is witnessing a rise in violence fed by political and social discontent two years after the mass uprising that toppled the former dictator Zine El Abidine Ben Ali.