Mr Karadzic told the judges he did not plan to testify in his own defence
Former Bosnian Serb leader, Radovan Karadzic, has cross-examined the first prosecution witness at the resumption of his war crimes trial at The Hague.
Ahmet Zulic, a Bosnian Muslim, told the judges his elderly father-in-law had been burned alive by Serb forces.
Mr Karadzic, defending himself, asked Mr Zulic a long list of questions and was told to "come to the point".
Mr Karadzic denies 11 charges of genocide, war crimes and crimes against humanity over the 1992-95 Bosnian war.
In his opening statement last month, Mr Karadzic described the conflict as "just and holy", and blamed Bosnian Muslims for starting it.
Up until then, he had boycotted the trial and sought to delay it.
Prosecutors at the International Criminal Tribunal for the former Yugoslavia (ICTY) are expected to present evidence from some 410 witnesses, over several months.
Peter Biles, BBC News, The Hague
The appearance of the first prosecution witness means that Radovan Karadzic's trial is now properly under way. In spite of his demands over the last few months for more time to prepare his case, the former Bosnian Serb leader was fully engaged as he began the cross-examination of the witness, Ahmet Zulic.
There were tetchy exchanges between the two men as Mr Karadzic sought to test the credibility of Mr Zulic. Before the tribunal adjourned, Judge Howard Morrison gave Mr Karadzic some words of advice on how to conduct his own defence. It amounted to a mini-tutorial in the skills of legal cross-examination. Mr Karadzic was advised to be more concise, to avoid passing comment when questioning the witness and to be wary of leading questions.
Earlier there was a signal that Mr Karadzic was again trying to prolong the trial. Judge O-Gon Kwon said that Mr Karadzic was asking for four or five times as many hours as the prosecution wanted, for questioning witnesses.
In his testimony, Mr Zulic said his father-in-law had been killed by Serb troops "mopping up" the survivors of an artillery attack in May 1992 on the mainly Muslim village of Mahala, near Sanski Most in north-western Bosnia.
"You could see houses burning," he said. "People who were infirm remained in Mahala... because they had no time to escape.
"I could see shells flying and exploding. You could see the houses burning at night."
When asked who was responsible for the attack, Mr Zulic responded: "It was the Serb army of occupation and Serb paramilitary units."
Mr Zulic also watched Serbs force about 20 Muslim men to dig their own graves before slashing their throats or shooting them, prosecutor Ann Sutherland said.
He was later held in Bosnian Serb-run detention centres from June to November 1992, during which time his weight dropped from 90kg to 55kg (198 to 121lbs), she added.
Describing his imprisonment, Mr Zulic told the court: "Two men would kick us in one part of the body and another would use a baton to beat you over the head until you became unconscious."
"I had fractures, broken ribs - six or seven vertebrae were affected."
WITNESS - AHMET ZULIC
Previously gave evidence in three other trials, including that of former Yugoslav President Slobodan Milosevic
Muslim former inmate of Manjaca detention camp, near Sanski Most
Testimony includes alleged murder in June 1992 of about 20 men made to dig their own graves by - it is claimed - Serb soldiers
Also a witness to alleged prisoner beatings and starvation
Mr Karadzic asked Mr Zulic a long line of questions about his political allegiance and whether there was an organised resistance among Bosnian Muslims to Serbs.
Mr Karadzic was told to "come to the point" by Presiding Judge O-Gon Kwon and warned a time limit could be imposed on his cross-examination of witnesses.
The BBC's Peter Biles at The Hague says that so far there has been no evidence from Mr Zulic linking these events to Mr Karadzic.
Mr Zulic has given evidence to the ICTY on three previous occasions, including in the trial of former Yugoslav President Slobodan Milosevic, who died of a heart attack in 2006 before his trial concluded.
At the beginning of Tuesday's proceedings, Mr Karadzic told the court that he did not plan to testify in his own defence, which would deprive prosecutors the chance to cross-examine him.
Mr Karadzic faces two charges of genocide, as well as nine other counts including murder, extermination, persecution and forced deportation.
Eleven counts of genocide, war crimes, crimes against humanity and other atrocities
Charged over shelling of 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
Prosecutors say he orchestrated a campaign of "ethnic cleansing" against Muslims and Croats in eastern Bosnia to create an ethnically pure Serbian state.
In his opening statement last October, prosecutor Alan Tieger said Mr Karadzic had "harnessed the forces of nationalism, hatred and fear to pursue his vision of an ethnically segregated Bosnia".
Mr Karadzic had boycotted earlier proceedings, insisting on more time to prepare his case. His last attempt to have the trial postponed was dismissed by the Appeals Chamber this month.
In November, the court appointed British lawyer Richard Harvey to be present in court to represent him if that became necessary.
Mr Karadzic was arrested in Belgrade in 2008 after nearly 13 years on the run.
During his time in power, he was president of the self-styled Bosnian Serb Republic and commander of its army during the Bosnian conflict which left more than 100,000 people dead. He is the most significant figure to go before the ICTY since Milosevic.
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