From his cell in Serbia where he is waiting to be transferred to a UN war crimes court in The Hague, Mladic called for calm, his lawyer said.
"He is appealing to people to calm down, there should be no bloodshed, he does not want to be a cause of unrest," lawyer Milos Saljic told reporters on Saturday.
|Serbia is bracing for protests against the arrest of Bosnian Serb war crimes suspect Ratko Mladic, pictured, amid fears of an outbreak of ultra-nationalist street violence|
"He appeals for there to be no riots, for everything to be peaceful," Saljic said.
The authorities have nonetheless stepped up security before the protest, called by the ultra-nationalist Radical Party (SRS) from 7:00 pm (1700 GMT), though they have promised it will be allowed to go ahead.
"The police will not use force in advance... only if there is a drastic violation of public peace and order," Interior Minister Ivica Dacic told journalists.
"Security measures have been increased to a higher level," he said, including around state buildings and embassies.
After the arrest of Bosnian Serb wartime political leader Radovan Karadzic in July 2008, thousands of ultra-nationalists violently protested in Belgrade, leaving one dead.
Mladic, the head of Bosnian Serb forces during the 1992-95 Bosnian war, is still considered a hero by many in Serbia.
Accused of masterminding the 1995 Srebrenica massacre and other atrocities during the war, Mladic was arrested Thursday and is set to be transferred to the Hague-based International Criminal Tribunal for the former Yugoslavia (ICTY).
His lawyer is expected to launch an appeal on Monday against a ruling that he is fit to stand trial and be transferred to the international court, where he faces charges of war crimes, genocide and crimes against humanity.
SRS lawmaker Vjerica Radeta insisted Sunday's rally would be peaceful.
"I don't expect any incidents," Radeta told B92 television. "Our call to citizens, to our members and to supporters of the Serbian Radical Party was public and it was an invitation to come to a peaceful demonstration."
Far-right group 1389 also called for its supporters to join Sunday's rally.
"Let us 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," it said in a statement.
Another protest organised by an association of Bosnian Serb ex-soldiers was due to begin at mid-day Sunday in the Bosnian village of Kalinovik, where Mladic was born.
The ICTY indictment holds Mladic responsible for the Srebrenica massacre, Europe's deadliest atrocity since World War II, in which 8,000 Muslim men and boys were rounded up and killed.
It also accuses him of having overseen the 44-month siege of the Bosnian city of Sarajevo, during which 10,000 were killed.
Serbia has also vowed to track down those who helped protect Mladic during his 16 years on the run, amid questions over why it took so long for him to be captured.
News of Mladic's arrest was largely welcomed around the world, but some small protests have erupted in the region following his detention.