Serbian Prime Minister Zoran Djindjic was on Wednesday morning assassinated outside a government building in capital Belgrade. He was hit by two bullets.
A long time opposition leader, Djindjic had played a key role in toppling the regime of Yugoslav president Slobodan Milosevic.
This may have earned him the ire of Milosevic's supporters.
Djindjic's pro-West stance is also being cited as a possible reason for the attack.
Earlier, Deputy Prime Minister Nebojsa Covic told reporters that Djindjic had been shot in the chest and stomach at 1655 IST and died at 1800 IST.
Heeding the government's call, acting president Natasa Micic declared a state of emergency, which is expected to be in force until the perpetrators of the assassination are caught.
The government has also announced a three-day period of mourning.