Over 5,000 people are on a 110km-long march along mountain paths in Bosnia to commemorate the Srebrenica massacre of nearly 8,000 Muslims by Serbs.
The marchers started from the northeast Bosnian village of Nezuk on Thursday and will walk into Srebrenica on Sunday for the ceremony
marking the 15th anniversary of the killings.
The ceremony will include the burial of 775 recently identified victims at the special memorial centre and graveyard in Potocari near Srebrenica, joining the 3,749 already there.
Bosnian Muslim men and boys were systematically killed in the days following the fall of the Srebrenica enclave, designated a UN safe area, to Bosnian Serb troops on July 11, 1995.
Tens of thousands of people are expected to attend the commemoration, including Boris Tadic, the Serbian president, Ivo Josipovic, his Croatian counterpart, and Recep Tayyip Erdogan, the Turkish prime minister.
This year's commemoration of the worst single atrocity on European soil since World War II comes three months after the Serbian parliament adopted a resolution condemning the massacre and apologising to the victims.
The resolution, which ended years of denials from Belgrade about the scale of the massacre, was criticised by victims' organisations because the parliament did not use the term genocide.
The presence of Tadic at the commemoration remains a sore point for many survivors.
"It's good that he's coming, whatever his reasons, but I will ask him why he has not brought Ratko Mladic with him," Munira Subasic, who heads an organisation of Srebrenica women, said.
Mladic, the wartime commander of the Bosnian Serb military, is wanted for genocide, war crimes and crimes against humanity for his role in the Srebrenica massacre. He has been on the run for nearly 15 years and is believed to be hiding in Serbia.
Bringing to justice
Radovan Karadzic, the alleged mastermind behind the Bosnian Serb campaign of
ethnic cleansing and the Srebrenica killings, was arrested in Belgrade in 2008.
He is currently on trial for genocide before the International Criminal Tribunal for the former Yugoslavia (ICTY).
The ICTY has charged 21 people over the massacre.
On June 10 two former Bosnian Serb officers were sentenced to life in prison for their part in the massacre.
In 2004 Bosnian Serb ex-general Radislav Krstic, Mladic's right hand man who led the attack on Srebrenica, was sentenced to 35 years in prison for genocide.
Since the end of the war Bosnia has consisted of two semi-autonomous entities - the Serbs' Republika Srpska and the Muslim-Croat Federation, each with its own government.
The Republika Srpska government continues to contest the scale of the massacre, saying in April it would seek the revision of a 2004 report in which it accepted that more than 7,000 people were killed.
Milorad Dodik, the Bosnian Serb prime minister, now says the report was adopted under intense international pressure and maintains that 3,500 dead is a more likely figure.