The former Bosnian Serb leader, Radovan Karadzic, is due to be formally charged by the United Nations war crimes tribunal on Thursday.
Mr Karadzic led the formation of a separate Bosnian Serb assembly in 1991 - one of the sparks that ignited the 1992-1995 Bosnian war.
He has been was indicted on 11 counts of war crimes in connection with the 1990s Bosnian conflict, but the exact charges will only be revealed in court. He will be asked to enter a plea, but is under no obligation to do so.
Mr Karadzic was arrested in Belgrade on 21 July after 13 years on the run, and extradited to the Netherlands where he is being held at a detention centre.
The 63-year-old had changed his appearance, having grown a long beard, and was working as an alternative therapist. He is reported to have shaved his beard and had a haircut while he has been in custody.
Mr Karadzic will appear before the International Criminal Tribunal for the former Yugoslavia on Thursday and then will have 30 days to enter a plea. He was said by his lawyers to have been in a "relaxed and confident" mood before being extradited to the Netherlands on Wednesday.
Speaking at a news conference on Wednesday, the tribunal's chief prosecutor, Serge Brammertz, said Mr Karadzic's arrest was "immensely important", adding that the victims of the war had "waited too long for this day".
However, both prosecution and defence teams said they would need months to prepare for the trial.
In addition, Mr Karadzic has said that he does not recognise the court and plans to conduct his own defence. As such, this first court appearance likely to be just the opening skirmish in a long legal battle, the BBC reports.
On Wednesday at least 10,000 supporters held a rally to protest Mr Karadzic's arrest.
A travel ban on his family was lifted on Wednesday, to allow them to visit him in The Hague.
Eleven counts of genocide, war crimes, crimes against humanity and other atrocities.
Charged over shelling Sarajevo during the city's siege, in which some 12,000 civilians died.
Allegedly organised the massacre of up to 8,000 Bosniak men and youths in Srebrenica.
Targeted Bosniak and Croat political leaders, intellectuals and professionals.
Unlawfully deported and transferred civilians because of national or religious identity
Destroyed homes, businesses and sacred sites.