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Chokri Belaid was shot at close range as he left for work by a gunmen who fled on the back of a motorcycle; crowds poured on to the streets of Tunis and other cities, attacking offices of the main ruling party Ennahda, and by the end of the day the Islamist prime minister promised a national unity government.
There was no immediate local reaction to the plan by Prime Minister Hamadi Jebali of Ennahda to dissolve his coalition and bring in a wider range of political groups. After dark, hundreds of demonstrators were still fighting running battles with police in the capital, throwing rocks amid volleys of teargas.
Jebali, whose party has dismissed any suggestion it might be behind the assassination, said he would shortly announce the formation of a new government of non-partisan technocrats.
World powers, alarmed in recent months at the extent of radical Islamist influence and the bitterness of the political stalemate, urged Tunisians to reject violence and see through the move to democracy they began two years ago, when the Jasmine Revolution ended decades of dictatorship and inspired fellow Arabs in Egypt and across North Africa and the Middle East.
As in Egypt, the rise to power of political Islam through the ballot box has prompted a backlash among less organised, more secular minded political movements in Tunisia. Belaid, a 48-year-old left-wing lawyer who made a name challenging the old regime of Zine al-Abidine Ben Ali, led a party with little electoral support but his vocal opinions had a wide audience.
The day before his death he was publicly lambasting a "climate of systematic violence". He had blamed tolerance shown by Ennahda and its two, smaller secularist allies in the coalition government toward hardline Salafists for allowing the spread of groups hostile to international culture.