Acting Serbian Prime Minister Nebojsa Covic said yesterday that the police had arrested around 40 people in connection with the assassination of Prime Minister Zoran Djindjic.
"Around 40 people have been arrested, we will see what will come out of that, what they will say, what information will emerge," he said in an interview aired on B92 television.
Police and government officials earlier said key suspects remained at large. The government has said a Belgrade-based criminal group was behind Wednesday's assassination and named 20 of its alleged leaders.
Covic also confirmed media reports that police had brought in two Milosevic-era state security chiefs, Jovica Stanisic and Franko Simatovic, for questioning.
Meanwhile, European Union president Greece gave assurances that the 15-nation bloc would support Belgrade in its attempts to shore up stability in Serbia and the Balkans.
European foreign policy chief Javier Solana flew to Belgrade yesterday to express the EU's continued political and financial support for Serbia, which aspires to join the bloc.
Political analysts said Serbia had to move swiftly to crush the criminal gangs behind the murder or face chronic instability which will ruin its chances of rejoining Europe's mainstream.
Hundreds of mourners filed by the government building in central Belgrade, laying flowers and lighting candles. Djindjic's funeral will be held in Belgrade tomorrow.
"It is a wake-up call, especially for those in the West who have failed to acknowledge the power of organised crime in southeast Europe. (Balkan) organised crime is already extending its tentacles into the European Union," Misha Glenny, author of books on the former Yugoslavia and the Balkans, told Reuters.
Glenny played down talk of a descent into civil war, but experts said a prolonged power vacuum could spark more political violence, scare off much-needed foreign investment and drag down the rest of the Balkan region.
Korac vowed Serbia would press on with reforms to snuff out the mafia-like culture spawned under Milosevic's turbulent rule.
The government accused Belgrade-based criminal gang Zemun for the killing and named around 20 of its alleged leaders.
Korac said one of the prime suspects was an associate of Serbia's state security service who had taken part in a number of assassinations and that another was a former commander of a battle-hardened special police unit.
The government announced three days of mourning for Djindjic - the first European government leader to be killed since Swedish Prime Minister Olof Palme in 1986.
It also declared a state of emergency, under which the army takes over police functions and civil rights are restricted.
Franz-Lothar Altmann of Germany's Stiftung Wissenschaft und Politik raised the prospect of a power struggle. "A long period of instability could endanger everything, above all economic stabilisation, if a credible successor is not swiftly found."
Western leaders, who have deployed thousands of Nato peacekeeping troops across the former Yugoslavia since the Balkan wars of the 1990s, accused extremists of trying to return the region to the chaos of the Milosevic era.
Among suspects of the alleged Zemun clan leaders was Milorad Lukovic, a former head of the "Red Berets" - a special police unit which fought in the wars in Croatia, Bosnia and Kosovo.
Also listed was Dejan Milenkovic, accused by police of trying to kill Djindjic last month with a truck that swerved towards the prime minister's convoy of cars.
Belgrade police chief Milan Obradovic said one assassin with a sniper rifle and two with hand guns took part in Djindjic's killing. One of Djindjic's bodyguards was seriously wounded.
A legal source said separately that police had brought in two Milosevic-era state security chiefs, Jovica Stanisic and Franko Simatovic, for questioning related to the murder.