Ahmet Zulic witnessed Serb soldiers ordering Muslim men to dig their own graves before shooting them dead or slashing their throats; he endured incarceration in a Serb-run detention camp where he was savagely beaten and other Bosnian Muslim men around him died agonising deaths; he was forced to drink his own urine to survive.

Yesterday, 15 years after the Bosnian war ended, Mr Zulic became the first person to testify at the war crimes trial of Radovan Karadzic, the former Bosnian Serb leader accused of leading a genocidal campaign to make Bosnian Muslims, in the words of the prosecution, "disappear from the face of the earth".

In chilling testimony, Mr Zulic quietly told the court how he was brutally beaten at the notorious Manjaca camp. "They beat me nearly to death... I had horrific injuries. If I wanted to show you I would have to take my clothes off," he told the court, speaking through an interpreter. "I was in terrible pain, there was water in my lungs."

"I am physically disabled ... psychological stress and burden is catching up with me and getting worse. Even now, as I am speaking, I don't know what it is going to be like for me when I get home because every time I tell it I relive it again and again."

After being taken prisoner along, he was initially kept in a concrete garage with up to 90 other Bosnian Muslim men. Serb soldiers stamped on his fingers when he refused to make the sign of the cross, he told the court. Other prisoners were viciously attacked. "I could hear screams and cries during the night all the time," he said.

Dressed smartly in a suit and pink tie for his appearance at the court in the Hague, Mr Zulic recalled how he saw men around him die while crammed into stifling conditions in covered trucks while being transported to another detention camp.

"I remember two brothers ... they took 10 minutes to die, which seemed to be a whole eternity to me. Some died silently - they just didn't have enough air to breathe. I had to drink my own urine because there was no water," he said.

The prosecution says Mr Zulic was held in Bosnian Serb-run detention facilities from June to November 1992, during which time his weight dropped from 90kg to 55kg.

Mr Zulic testified seeing Bosnian Serb forces shell villages in northern Bosnia, and set houses on fire as prisoners were shot and had their throats cut. His accounts were intended to illustrate the savage campaign by Bosnian Serbs early in the 1992-95 war to carve out an ethnically pure Serb mini-state in Bosnia.

Mr Karadzic, conducting his own defence, also cross-examined Mr Zulic, who, at times, replied defiantly to questions and upbraided the defendant for interrupting him. The judge also repeatedly urged Mr Karadzic to move on following rambling questioning.

The witness statements have been delayed for months after Mr Karadzic boycotted the start of his trial last year, claiming he needed extra time to prepare his defence. The former Bosnian Serb leader faces 11 charges of genocide, crimes against humanity and violating the laws and customs of war stemming from the 1992-95 Bosnian war. He denies them all and faces a life sentence if convicted.

Mr Karadzic's case is the Yugoslav war crimes tribunal's most important since the country's former president Slobodan Milosevic died in his jail cell in 2006 before judges at the UN court could reach a verdict in his trial. The cases cover many of the same atrocities.

In his opening statement to the court on 1 March, Mr Karadzic denied involvement in the four-year siege of Sarajevo by Serb forces, where 10,000 people died, and the massacre of more than 7,000 Bosnian Muslim men and boys at Srebrenica in 1995.