A state of emergency has been been declared in Serbia after the assassination of the Prime Minister Zoran Djindjic.
World leaders have expressed shock and outrage at the killing.
Mr Djindjic, who was 50, was taken to hospital after being wounded with two bullets in the stomach and back outside government offices in Belgrade at around 12.45pm.
Private television station BK earlier reported that he had been operated on and that his life was not in danger.
The radio said police had arrested two people following the attack.
A reformist who became the head of Serbia's first non-communist government in January 2001, Mr Djindjic escaped a previous assassination attempt in February, when a lorry cut in front of his car.
He was a key figure in organising mass demonstrations against former Yugoslav leader Slobodan Milosevic from 1996 to 2000.
He authorised the transfer in 2001 of the former president to The Hague, where Mr Milosevic now faces life in prison on charges including genocide and crimes against humanity.
Mr Djindjic's close friendship with Vojislav Kostunica, who succeeded Milosevic, evolved into a bitter political rivalry.
Djindjic wanted Serbia to put its faith in the market economy, and relentlessly pushed his compatriots toward integration with the West after Yugoslavia became a pariah state in the latter years of Milosevic's rule.
