A war crimes trial of former Bosnian Serb army commander Ratko Mladic is to resume, with the prosecution focusing on the Srebrenica massacre in 1995.
Gen Mladic is accused of orchestrating the killings of more than 7,000 Bosnian Muslim boys and men in the town.
In all, the 70-year-old faces 11 charges, including genocide, during the brutal 1992-95 Bosnian war.
He calls the claims "monstrous", and the court in The Hague has entered a not guilty plea on his behalf.
The Srebrenica massacre was the worst atrocity in Europe since the end of World War II.
Serb fighters overran the enclave in eastern Bosnia - supposedly under the protection of Dutch UN peacekeepers. Bosniak (Bosnian Muslim) men and boys were separated off, shot dead and bulldozed into mass graves - later to be dug up and reburied in more remote spots.
Gen Mladic is also charged in connection with the 44-month siege of Sarajevo during which more than 10,000 people died.
On the first day of the trial on Wednesday, the prosecution at the International Criminal Tribunal for the former Yugoslavia (ICTY) argued that Gen Mladic intended to "ethnically cleanse" Bosnia.
It opened the hearing with an audio-visual presentation laying out the case.
Prosecuting counsel Dermot Groome said they would prove Gen Mladic's hand in the crimes.
"Four days ago marked two decades since Ratko Mladic became the commander of the main staff of the army of Republika Srpska - the VRS," he said.
Mr Groome said that by the time Gen Mladic and his troops had "murdered thousands in Srebrenica", they were "well-rehearsed in the craft of murder".
He then showed judges video of the aftermath of a notorious shelling of a market in the Bosnian capital Sarajevo, in which dozens of people died.
Mr Groome said there was "no doubt" that Gen Mladic had controlled the shelling of Sarajevo. He had promised that the city would shake, the prosecutor said.
Mr Groome said the attacks were part of an "overarching" plan to ethnically cleanse non-Serbs from parts of Bosnia.
He said crimes of sexual violence had played an integral part of the process of "taking over and ethnically cleansing Bosnia".
The prosecution also highlighted the role of snipers in Sarajevo, showing images of a child shot dead on a street and pictures taken from sniper nests overlooking the besieged city.
During the proceedings, members of the Mothers of Srebrenica group held a vigil outside the court.
Judicial authorities have rejected defence calls to delay proceedings, most recently a petition to have Dutch Judge Orie replaced on grounds of alleged bias.
However, even as the trial began, there were further indications it would be delayed.
Judge Orie said the court was considering postponing the presentation of evidence - due to start on 29 May - due to "errors" by the prosecution in disclosing evidence to the defence.
Mr Groome said he would not oppose a "reasonable adjournment".
Gen Mladic spent 15 years on the run before being apprehended by Serb forces last May and sent to The Hague.
The number of crimes of which he stands accused has been almost halved to speed up his trial.
Gen Mladic is accused of committing genocide and other crimes against Bosniaks (Bosnian Muslims) and Bosnian Croats in a campaign of ethnic cleansing that began in 1992 and climaxed in Srebrenica.
Pre-trial hearings have been characterised by ill-tempered outbursts from Gen Mladic, who has heckled the judge and interrupted proceedings.
"The whole world knows who I am," he said at a hearing last year.
"I am General Ratko Mladic. I defended my people, my country... now I am defending myself."
The case has stirred up strong emotions among watching survivors, with some shouting "murderer" and "killer" from the court gallery.
However, while Gen Mladic's critics consider him a butcher, to some Serbs he is a national hero.
Gen Mladic suffered at least one stroke while in hiding and remains in frail health.
The architect of the Balkan wars, former Serbian President Slobodan Milosevic, died in detention in his cell in 2006, before receiving a verdict.