Twenty-one people arrested in connection with the 12 March murder of the reformist leader who helped oust Slobodan Milosevic, began their trial in a high security special court on Monday.
They include the former Red Berets deputy commander Zvezdan Jovanovic, arrested on suspicion he pulled the trigger outside the government headquarters.
Jovanovic, 38, showed little emotion as he was led handcuffed into the specially built courtroom in Belgrade and sat down near other indictees behind bulletproof glass.
Fifteen indicted suspects on the run are being tried in absentia, including a war-hardened former police commander, accused of masterminding the killing that shocked the West and weakened the ruling coalition.
Former premier Zoran Djindjic
was shot and killed on 12 March
Djindjic, 50, infuriated Serb nationalists by sending Milosevic to The Hague war crimes court. Authorities blame his killing on criminal gang bosses linked to a crack police unit set up under Milosevic's rule.
Many people in the country see the UN tribunal as anti-Serb.
The trial has begun just a few days before a 28 December general election pits Djindjic allies against resurgent ultra-nationalists in a vote that could determine the fate of his pro-market reforms.
'Mastermind' at large
But the trial will begin with the suspected mastermind, former Red Berets chief Milorad "Legija" (Legion) Lukovic, still at-large.
Lukovic got his nickname after serving with the French Foreign Legion in the 1980s. He later fought in the Serbian-instigated Balkan wars.
A special court has been set up to
deal with organised crime
"He's certainly the key witness, since he was an organiser of the entire action," Deputy Prime Minister Zarko Korac told the daily Vesti on Sunday.
"It is hard to believe that he has done everything on his own without political support," Korac added, echoing the view of other officials suspecting a deeper conspiracy.
Fifteen people were indicted as directly involved in Djindjic's assassination among 36 people charged with murder, terrorism and criminal conspiracy, plus 14 other killings, three abductions and other criminal acts.
Mafia-style crime, linked to state security, flourished under Milosevic when Serbia was isolated internationally. Officials had said Djindjic was planning a crackdown when he was killed.