She took the stand shortly after former US Secretary of State Madeleine Albright spoke in her support at the war crimes tribunal in The Hague.
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You must strive to find whatever justice this world can offer, not only for me, but also for the innocent victims of this war
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Biljana Plavsic
Mrs Plavsic, has pleaded guilty to crimes against humanity and faces possible life imprisonment.
In a statement she read on Tuesday, she admitted that "many thousands of innocent people were victims of an organised effort to remove Bosnian Muslims and Bosnian Croats from territory claimed by Serbs".
"At the time I convinced myself that this [was] a matter of survival and self-defence, but [that] explanation... offers no justification," she said.
Supported Dayton
Mrs Plavsic is the highest-ranking Serb leader to admit to crimes against humanity and the only woman to appear before the tribunal.
"You must strive in your judgement to find whatever justice this world can offer, not only for me, but also for the innocent victims of this war," she told the three-judge panel.
Mrs Albright said that towards the end of the Bosnian war in 1992-1995 Mrs Plavsic had split from the Serb hardliners and stood up for the Dayton peace accords.
Mrs Albright - who at the time was the US ambassador to the United Nations - also spoke of the "unimaginable horrors" of the Bosnian war "unseen in Europe since World War II".
WITNESSES
"Obviously, she [Plavsic] was involved in horrendous things," Mrs Albright said.
She also said Mrs Plavsic took political risk in supporting the Dayton agreement, and that this in the end cost her the Bosnian Serb presidency.
Mrs Albright was Secretary of State from 1997 to 2001 and met Plavsic in 1997. She was appearing both for the prosecution and the defence.
Carl Bildt - who served as the first international envoy to Bosnia after the war ended - was called by the defence alone.
"She wanted to preserve the possibility of Serbs, Croats and Muslims living together in the future," he said.
Mrs Plavsic was a deputy of wartime Bosnian Serb leader Radovan Karadzic, who is now at the top of the prosecutors' wanted list.
She was originally charged with genocide, persecution, extermination and deportation - but all but one charge was dropped after she pleaded guilty to persecution.
Her lawyers said her changed plea showed "her remorse fully and unconditionally".
Milosevic role
On Monday, Mrs Plavsic said Radovan Karadzic and Slobodan Milosevic, then Serbian president, were the masterminds of the ethnic cleansing plan.
Her plea states that Bosnian Serb political leaders "frequently went to Belgrade to consult with, take guidance from or arrange support from Milosevic".
However this information may not help prosecutors involved in Mr Milosevic's trial, who are seeking to establish a link between him and the Bosnian Serb leadership.
Mrs Plavsic has made clear that she will not testify at other trials.