Serbian authorities have banned gatherings in Belgrade to commemorate the Srebrenica massacre, after right-wing groups threatened disruption.
Saturday is the 20th anniversary of the massacre, in which some 8,000 Muslim men and boys were killed by Bosnian Serb forces during the Bosnian war.
Activists had planned to mourn the victims outside the Serb parliament.
The official memorial ceremonies will take place on Saturday in Srebrenica itself.
Serbian police responded to plans for counter-demonstrations by banning all gatherings at parliament on Saturday.
Serbia's Interior Minister Nebojsa Stefanovic said he had taken the decision to guarantee "peace and security in the whole of Serbia".
But the initiator of the planned commemoration in the Serbian capital, Dusan Masic, said it was "scandalous", and showed "where Serbia is in 2015".
Balkanist magazine tweeted that a commemoration would take place on Friday evening instead.
Serbia's Prime Minister Aleksandar Vucic plans to travel to Bosnia for the official memorial events in Srebrenica.
The atrocity took place on 11 July 1995, amid the bloody break-up of Yugoslavia into independent states.
The Bosnian War saw Bosnian Serb forces, backed by Serbia, fighting the Muslim-led Bosnian government.
Thousands seeking shelter at what was supposed to be a UN refuge in Srebrenica were shot ded.
On Wednesday, a United Nations Security Council resolution describing the Srebrenica massacre as "genocide" was vetoed by Russia.
The events at Srebrenica were the worst atrocity on European soil since World War Two.
Fourteen individuals have been convicted at a UN tribunal in The Hague in relation to the Srebrenica killings.
The former Bosnian Serb army chief, Ratko Mladic, and former Bosnian Serb leader Radovan Karadzic are both on trial at The Hague in separate processes.
They are each accused of crimes relating to the Srebrenica massacre.