Relatives of the victims will be joined by dignitaries including former US President Bill Clinton.
There will also be a funeral service for more than 100 victims whose remains were recently identified using DNA analysis.
On Friday, almost 10,000 people took part in an annual peace march, retracing the route of the men and boys who tried to flee Srebrenica as Bosnian Serb forces took control of the town.
Bill Clinton, the US president at the time, will be joined by Turkish Prime Minister Ahmet Davutoglu and the Serbian Prime Minister Aleksandar Vucic for Saturday`s ceremony. EU foreign policy chief Federica Mogherini will also attend.
The Bosnian War saw Bosnian Serb forces, backed by Serbia, fighting the Muslim-led Bosnian government.
In July 1995, thousands seeking shelter in what was supposed to be a UN refuge in Srebrenica were shot dead. It was Europe`s worst atrocity since World War Two.
In the run-up to the anniversary, Serbian authorities banned commemorations in the capital Belgrade after right-wing groups threatened disruption.
Activists had planned to mourn the victims outside the Serb parliament but 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".
The initiator of the planned Belgrade commemoration, Dusan Masic, said it was "scandalous", and showed "where Serbia is in 2015".
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 both accused of crimes relating to the massacre.
More about:
