Visiting Serbian Prime Minister Aleksandar Vucic tried to join other politicians paying respects at the Srebrenica graveyard, where more than 100 newly found remains were to be buried with 6,000 other massacre victims.
As he walked to the site, people hissed and yelled, unprepared to accept an official from a country that once directed the Bosnian Serbs militants.
"Takbir!" a man in the crowd shouted. "Allahu akbar!" ("God is great") the crowd responded repeatedly, gradually getting louder.
As Vucic made his way into the graveyard, people tossed rocks and bottles at him. His dark-suited security staff rushed him up the graveyard's steep hill, among the victims' gravestones.
At the top, the staff ushered him into his car as objects continued to fly, and a driver hurriedly spirited him away.
The scene came days after Russia, a Serbia ally, vetoed a U.N. Security Council measure that would have labeled the massacre as genocide
Vucic returned to Belgrade, where he told reporters that a stone hit him in the mouth, but that he was OK.
"I regret that some people haven't recognized my sincere intention to build friendship between Serbian and Bosniak people," he said. "... I still give my hand to the Bosniak people. I will continue with that ... and always be ready to work together to overcome problems."
Vucic's presence Saturday earned a statement of empathy from the European Union's foreign policy chief, Federica Mogherini, who attended the commemoration.
"My solidarity to @avucic who made the historical choice of being present in #Srebrenica," she posted on Twitter. "Peace can be built only on reconciliation."
A statement from the U.S. Embassy in Bosnia-Herzegovina condemned the incident.
"Many of the mourners were horrified by the violence and disheartened that it disrupted the solemnity of the anniversary," the statement said. "The U.S. Embassy in Sarajevo shares that sentiment, and urges all to focus once again on the victims of the Srebrenica genocide and their families."
Earlier, Mogherini and other dignitaries, including former U.S. President Bill Clinton, spoke of how the massacre -- begun 20 years ago to the day -- should serve as a lesson to the world.
For Europe, Srebrenica is a painful reminder of a continent not having acted quickly enough to stem the bloodshed.
"In Srebrenica, Europe is faced with its shame," Mogherini said.
"Europe failed to stand up to the promise of our founding fathers and to the dreams of their grandsons: no more war in Europe, no more murders in the name of race or the nation. No more genocides."
Starting on July 11, 1995, for three days, ethnic Serb forces gunned down Muslim boys and men in and around Srebrenica. The sight of their broken bodies dumped into mass graves, belongings lining roadsides, and carnage strewn across fields forced the world's eyes onto a broader campaign of ethnic cleansing.
This came toward the end of the Bosnian war, in which the Bosnian Serb army -- largely ethnic Serbs who once served in the Yugoslav army -- fought to carve out its own territory within Bosnia following Yugoslavia's breakup. The extended slaughter of civilians, including children, women and old men in Bosnia-Herzegovina, eventually claimed around 100,000 lives; most of the dead were Muslims.
Prior to Srebrenica, the United Nations had warned for months that the Bosnian Serbs were poised to move again, with ruthless consequences for Muslim civilians.
"That awful act finally stirred all the members of NATO to support the military intervention that was clearly necessary," Clinton said in his address Saturday.
Weeks after Srebrenica, NATO jets bombed Bosnian Serb positions for two weeks in Operation Deliberate Force. The Serbs quickly surrendered, and months later both sides signed an accord worked out in Dayton, Ohio, establishing a peace that has lasted since.
But the bodies of the dead were barely covered as they sat in various spots, and searchers still work to recover and identify them.
136 more buried Saturday
While crowds strolled Saturday between obelisk-shaped grave markers covering the memorial site, some paused to read inscriptions or pray.
Freshly dug graves gaped at passersby, waiting for newly found bodies to fill them as part of the commemoration. Coffins wrapped in smooth, green cloth lay in rows up and down a path.
"Today, loved ones and total strangers from all over the world come here, and they can see that 6,000 men and boys are buried with more coming today," Clinton said. When he attended the site's inauguration in 2003, only 600 had been buried.
Srebrenica's mayor said 136 victims would find a final resting place Saturday, two decades after their killing.
There will be many funerals to come.
Monument against cruelty
The Srebrenica memorial cemetery has become an international monument against a repeat of such carnage.
"We owe the people that sacrificed their lives here," Clinton said.
The world must work to live up to the mission, Clinton said, as many people are still being killed over their ethnicity.
The war crimes led to the prosecution of former Serbian leaders in international courts. But many are not yet satisfied that justice has been done completely.
"Those who perpetrated that massacre must be brought to justice," U.S. Secretary of State John Kerry said in a statement. "And we must remember Srebrenica's victims and all the victims of the war -- not just today but every day."
Russian U.N. veto
But old resentments galvanized by war have a way of hanging on.
Parties to massacre, and their allies, sometimes object to the killings being called genocide, and with Russia's veto this week, the United Nations was unable this week to muster the support in the Security Council to lend that term to the ethnic carnage in Srebrenica.
On Thursday, the European Parliament criticized the vote. "MEPs (parliamentarians) regret that the U.N. Security Council failed to pass a resolution commemorating the genocide and call for acceleration of war crimes prosecution at international and domestic level," it said in a statement.
Two U.N. judicial bodies, the International Criminal Tribunal for the Former Yugoslavia and the International Court of Justice, both recognize the massacre as genocide and have condemned Serbia's ethnic cleansing campaign as "the biggest war crime in Europe since the end of the Second World War."
'Insults stated against Serbia'
Serbia, which is interested in joining the European Union, bristled at the criticism from Brussels.
"... The insults stated against Serbia regarding Srebrenica could in no way lead to a better future," its foreign ministry said in a statement.
And when Clinton and representatives of European Union countries took their places in front of the white pylon grave markers that stretch across the Srebrenica memorial cemetery, in neighboring Serbia there reportedly were no official commemorations, as these were canceled.
A group of protesters were expected to lie down in front of the Serbian national assembly building in Belgrade to honor the dead.