Thousands of people have gathered in central Belgrade for an antigovernment rally led by Serbian ultranationalist Vojislav Seselj.
Seselj announced plans for the demonstration one day after being released by the UN International Criminal Tribunal for the former Yugoslavia in The Hague on November 12.
He was allowed to return to Belgrade so the war crimes suspect could receive immediate treatment for cancer.
He has been held in detention in the Netherlands for 12 years after voluntarily surrendering himself to the tribunal.
Seselj, 60, called "patriots" to rally against efforts "to make Serbia join both the European Union and NATO."
At the rally, Seselj said that he will "destroy" The Hague tribunal, which he claimed is detaining Serbs "only because they are Serbs."
Serbian President Tomislav Nikolic and Prime Minister Aleksandar Vucic left Seselj's Serb Radical Party (SRS) in 2008.
Seselj calls them traitors and says the SRS will win the next elections because of its "economic and social arguments."
Seselj's trial on multiple charges ended in 2012 but no verdict has been delivered.
Seselj led his own defense before the Hague tribunal and regularly sought to obstruct the process. He has suggested he won't return if found guilty.
RFE/RL's correspondent and other local media said at least 3,000 people were at the November 15 rally; Reuters estimated the crowd at 5,000 people.
With reporting by AFP