The Serbian Government says 56 suspected members of a criminal group have been arrested following the assassination of the pro-reformist Serbian Prime Minister, Zoran Djindjic.
Djindjic played a leading role in ousting Milosevic
But Belgrade police chief Milan Obradovic said his troops were still looking for three key suspects, including the group's alleged leader - a former commander of a special police unit, Milorad Lukovic.
The arrests came during a state of emergency - under which some civil rights can be curtailed and the army takes over police duties.
Mr Djindjic was shot in the stomach and in the back outside government offices in Belgrade on Wednesday and later died of his wounds in hospital.
Hundreds of people have been queuing to sign a book of condolences near the scene of the murder. They have been laying flowers and lighting candles.
The government has declared three days of mourning. Flags are flying at half mast and commemoration ceremonies have been held in Serbia on Thursday.
They [the Zemun group] declared a war on our state and we declare war against them
Deputy Prime Minister
Mr Djindjic's funeral is to take place on Saturday.
Correspondents say the assassination of the prime minister heralds the start of turbulent days for Serbia, leaving the country with a potentially dangerous political power vacuum.
The Serbian Government has blamed the killing on the Zemun group - which takes its name from a Belgrade suburb.
It said three of the arrested had demanded a status of protected witness when testifying before Serbia's special anti-organised crime prosecutor.
As police intensified their manhunt, the country's Interior Minister Dusan Mihajlovic vowed to "arrest all responsible and liquidate anyone who resists arrest".
Deputy Prime Minister Zarko Korac went further.
CATALOGUE OF VIOLENCE
March 2003: Serbian premier Zoran Djindjic shot dead
Feb 2003: Djindjic says attempt made on his life
June 2000: Serb opposition leader Vuk Draskovic survives shooting
May 2000: Goran Zugic, national security adviser to pro-West Montenegrin president, shot dead
October 1999: Draskovic survives road accident "assassination attempt"
"They [the Zemun group] declared a war on our state and we declare war against them," Zarko Korac said.
"This is a group which is stronger than the authorities... which has, up to a certain point, contacts with the police," Mr Korac said.
"The most difficult thing is to erase organised crime which is linked with a police," he said.
The Serbian cabinet, which observed a minute's silence when it met for crisis talks after the attack, has told the press to publish only information from official government sources during the state of emergency.
Acting Serbian President Natasa Micic told Serbian Radio that she had declared the state of emergency "with the aim of safeguarding the security of people and property, and engaging in a determined showdown... with organised crime".
She urged people to remain calm, but said the state of emergency would remain in place until the killers were brought to justice.
'Bad day for the Balkans'
Vojislav Kostunica, former Yugoslav President and a long-standing rival of Mr Djindjic, said he was appalled by the attack.
"The fact that political violence is happening... is a terrible warning about how little headway we have made on the path of real democratisation of our society," he said just before Mr Djindjic's death was confirmed.
The European Union expressed shock and dismay at the assassination, with Greek Foreign Minister George Papandreou, whose country holds the EU presidency, sending condolences to Mr Djindjic's family "and to [the] entire Serb people".
People have been paying tribute
On 21 February, Mr Djindjic survived what he said was an assassination bid when a lorry swung into the path of his motorcade as he was travelling to Belgrade airport.
He later dismissed the incident as a "futile effort" which could not stop democratic reforms.
Correspondents say that Mr Djindjic, 50, made many enemies over his career as a pro-democracy campaigner and then as Serbia's prime minister.
He was pivotal in arresting and handing former President Slobodan Milosevic over to the war crimes tribunal in The Hague in June 2001.