As the top Bosnian Serb leaders of the time, Radovan Karadzic and Ratko Mladic are the alleged chief architects of the Srebrenica massacre. But, 20 years on, they have yet to be sentenced for Europe's worst atrocity since World War II.

Karadzic, the Serbs' political leader during Bosnia's brutal 1992-1995 civil war, and his military chief Mladic were both on the run for years before their separate arrests.

Now in their seventies, the pair are on trial at the International Criminal Tribunal for the former Yugoslavia (ICTY) for war crimes and crimes against humanity. The verdict against Karadzic is expected by the end of the year.

The charges include counts of genocide for the massacre in Srebrenica, the eastern Bosnian enclave where 8,000 Muslim men and boys were killed over several horrific days in July 1995.

Prosecutors say the pair, along with late Yugoslav president Slobodan Milosevic, tried to "cleanse" Bosnian Muslims and Croats from Bosnia's Serb-claimed territories after the former Yugoslavia's collapse in the early 1990s.

Both men are still considered heroes by many Serbs, but they are reviled as monsters by Bosnia's Muslims and Croats who blame them for the deaths of tens of thousands of people.

- 'Butcher of Bosnia' -

 Mladic, now 73, was notorious for his brutish wartime military leadership, but his name is associated above all with the slaughter at Srebrenica that began when his troops overran what was then a UN-protected enclave.

Two international courts have declared the mass killing a genocide.

Entering the deserted town on July 11, 1995, accompanied by a Serb TV crew, the military chief said: "We give this town to the Serb nation. Remembering the uprising against the Turks, the time has finally come to take revenge on the Muslims in this area."

He was filmed walking through the crowd of tens of thousands of scared Muslim refugees, handing out bread and giving chocolates to the children. There was "nothing to be afraid of", he assured them.

Hours later, his soldiers separated the men and boys, then drove them away.

Mladic began the war as a colonel in the Yugoslav National Army, and by May 1992 was commander of the Bosnian Serb forces.

Former Yugoslav army spokesman Ljubodrag Stojadinovic described him as "narcissistic, conceited, vain and arrogant", while a former colonel, Gaja Petkovic, said he was a "cynic and a sadist".

"Carried away by... fury and brutality, Mladic even invented completely new commands for his artillery units, such as: 'Clobber', 'Torch' and 'Beat them senseless'," Petkovic wrote in 1994.

Some reports suggest his daughter's suicide in 1994 pushed him over the edge, a year before he allegedly set the Srebrenica massacre in motion.

The Bosnian Serb government sacked Mladic in 1997, and he moved to the Serbian capital Belgrade in 2000.

At first he lived openly -- but as his popularity waned amongst Serbian politicians, increasingly concerned that failure to transfer Mladic to the UN war crimes court would further delay the country's joining the European Union, he went underground.

Mladic was finally arrested in 2011 at a relative's house in northern Serbia. The once stocky and ruthless soldier now cut a frail figure, dogged by health problems as the families of Bosnia's war victims waited anxiously for him to be brought to justice.

- 'Among most evil men' -

Karadzic, 70, has donned many guises: New Age healer, poet, psychiatrist, president and indicted war criminal.

Richard Holbrooke, a chief architect of the 1995 Dayton peace accord, described Karadzic as "one of the worst, most evil men in the world".

"He was the true believer in the racist theories," Holbrooke told German magazine Spiegel after Karadzic's arrest. "He believed in racial superiority... He would have made a good Nazi."

After nearly 13 years on the run, Karadic was arrested on a Belgrade bus in 2008 in his incarnation as "Dragan Dabic", a long-haired, bearded practitioner of alternative medicine.

He is accused of authorising ethnic cleansing in which more than a million non-Serbs were driven from the villages where they had lived for generations.

At his five-year trial, defiant Karadzic said he should be rewarded for doing everything to avoid war in Bosnia, insisting he had sought to reduce human suffering.

In his opening statement in 2010 Karadzic told judges that the atrocities blamed on Bosnian Serbs were "lies, propaganda and rumours" and that the Srebrenica massacre was a "myth".

A psychiatrist by training, Karadzic enjoyed writing children's poetry, theatre plays and Serb folk music in his spare time.

British general Michael Rose, who commanded UN peacekeepers in Bosnia in 1994, said Karadzic "was a consummate liar, inherently paranoid and a heavy drinker who plainly verged on alcoholism".