Ex-Bosnian Serb army chief Ratko Mladic says he did not order the Srebrenica massacre in 1995, according to his son.
Darko Mladic made the statement a day before his father is due to lodge an appeal against being transferred to the war crimes tribunal in The Hague.
Some 7,500 Bosnian Muslim men and boys were killed at Srebrenica, and the massacre is a key charge against Gen Mladic, 69, arrested on Thursday.
Thousands have gathered to support Gen Mladic in Serbia's capital Belgrade.
Darko Mladic spoke out after visiting his father, who is in detention at Serbia's war crimes court after 16 years on the run.
"He said that whatever was done in Srebrenica, he had nothing to do with it.
"He saved so many women, children and fighters... His order was first to evacuate the wounded, women and children and then fighters. Whoever did what behind his back, he had nothing to do with it."
'Regime of traitors'
To some Serbs Gen Mladic remains a national hero, and his son's statement came as more than 10,000 supporters of the general began protests in Belgrade to voice their opposition to his arrest and likely extradition.
Sunday's rally is taking place outside parliament in Belgrade.
Demonstrators listened to nationalist songs played by loudspeaker and waved flags describing Gen Mladic as a Serb hero.
Far-right group 1389 urged its supporters to "show to this regime of traitors that we are not afraid of their threats and repression and that we are ready to defend Serbian heroes".
An association of former Bosnian Serb soldiers held a separate protest against Gen Mladic's arrest in the Bosnian village of Kalinovik, where he was born.
The BBC's Nick Thorpe, in Kalinovik, said several thousand people had gathered and were protesting peacefully.
Gen Mladic's lawyer Milos Saljic has said his client knew he would be transferred to a UN war crimes tribunal in The Hague.
Mr Saljic is to appeal against the transfer on Monday, after a court said Gen Mladic was fit to be extradited.
Speaking on Sunday he maintained that Gen Mladic's health had deteriorated in the two days since the court's decision.
"I can tell you that his health condition today is much worse then yesterday. It is worse psychologically," the told the Associated Press.
Gen Mladic was seized in the village of Lazarevo, about 80km (50 miles) north of Belgrade.
Serbian officials have vowed to pursue those who helped him avoid detection.
Serbian war crimes prosecutor Vladimir Vukcevic told AFP: "By hiding Mladic they have caused serious damage to this country. Hiding fugitives from The Hague tribunal is a serious crime."
Following the arrest of Radovan Karadzic in 2008, Gen Mladic became the most prominent Bosnian war crimes suspect still at large.
He was indicted by the UN war crimes tribunal in The Hague in 1995 for genocide over the killings that July at Srebrenica - the worst single atrocity in Europe since World War II - and other alleged crimes.
Having lived freely in the Serbian capital, Belgrade, he disappeared after the arrest of former Yugoslav President Slobodan Milosevic in 2001.
Serbian President Boris Tadic has said the arrest brought the country and the region closer to reconciliation, and opened the doors to European Union membership for Serbia.