Two former close associates of ex-Yugoslav president Vojislav Kostunica, have been arrested as part of the investigation into Prime Minister Zoran Djindjic's murder.
Kostunica says the Djindjic tragedy is being used for political ends
Former army intelligence chief Aco Tomic and Rade Bulatovic, Mr Kostunica's security adviser, were detained late on Tuesday for alleged links with a criminal gang accused of organising the assassination.
The two men are the first officials close to Mr Kostunica to be arrested since the 12 March assassination.
Correspondents say the arrests could herald a crackdown on supporters of Mr Kostunica, who stepped down in February when Yugoslavia was replaced by the new state of Serbia and Montenegro.
Can you imagine anyone in America blaming the Republicans for John Kennedy's assassination?
Mr Kostunica replaced Slobodan Milosevic as president in October 1999 in an alliance with Mr Djindjic, but the two soon fell out over the pace of reform and whether to extradite war crimes suspects to The Hague.
Mr Kostunica has since stood unsuccessfully in two rounds of elections for the Serbian presidency.
A statement by the Serbian Government said Mr Bulatovic and Mr Tomic had had "meetings" and "arrangements" with the main organisers of Mr Djindjic's assassination.
But in an interview with the Blic daily, Mr Kostunica said he knew nothing of any such meetings and said the allegations against his colleagues were politically motivated.
"The tragedy of Djindic's assassination and the state of emergency are being used for a crackdown against political opponents," he said.
"Can you imagine anyone in America blaming the Republicans for [John] Kennedy's assassination?"
Nearly 8,000 people have been detained in the search for suspects in the assassination, and more than 2,000 are still in custody.
Police shot dead two of the suspected organisers of the assassination as they reportedly resisted arrest.
A third - Milorad Lukovic, the alleged leader of the Zemun criminal gang - is still at large.