Anger boiled over Saturday at a massive commemoration of the slaughter of Bosnia Muslims at Srebrenica 20 years ago as people pelted Serbian Prime Minister Aleksandar Vucic with stones, water bottles and other objects. An aide said the prime minister was hit in the face with a rock.

Vucic's associate, Suzana Vasiljevic, told The Associated Press that his glasses were broken when he was struck in the face with a stone. Vasiljevic said she was behind Vucic when "masses broke the fences and turned against us."

Tens of thousands came to mark the 20th anniversary of Europe's worst massacre since the Holocaust — the slaughter of 8,000 Muslims from the eastern Bosnian town of Srebrenica — with foreign dignitaries urging the international community not to allow such atrocities to happen again and to call the crime "genocide."

Vucic, once an ultra-nationalist, came to represent his country at the commemoration in an apparent gesture of reconciliation. He left the ceremony after coming under attack.

Serbia's foreign minister, Ivica Dacic, said that the attack on Vucic was an attack on Serbia. "By deciding to bow to the victims, Serbia's prime minister behaved like a statesman," Ivica Dacic said in a statement. "This is another negative consequence of politicizing this subject that has brought new divisions and hatreds instead of reconciliation."

Serbian Interior Minister Nebojsa Stefanovic described the incident as "an assassination attempt," accusing neighbor Bosnia of failing to "create even the minimal conditions for the safety of the prime minister."

As Vucic entered the cemetery to lay flowers, thousands booed and whistled. Someone threw a shoe at him, others threw stones, water bottles and other objects. Vucic and his guards then were forced to run through a crowd that rushed them. Vucic's guards tried to protect him with bags, umbrellas and their raised arms.

A few people carried banners with his own wartime quote: "For every killed Serb, we will kill 100 Bosniaks."

Serbia and Bosnian Serbs deny the killings were genocide, and claim that the death toll has been exaggerated.

Dozens of foreign dignitaries — including former U.S. President Bill Clinton, Turkish Prime Minister Ahmet Davutoglu, Britain's Princess Anne and Jordan's Queen Noor — came for the ceremony mourning the 8,000 Muslim men and boys killed in the eastern Bosnian town of Srebrenica by Bosnian Serb troops. The crime was later defined as genocide by two international courts.

"I grieve that it took us so long to unify ... to stop this violence," said former U.S. President Bill Clinton, who was in office at the time of the massacre and whose administration led the NATO airstrikes against Serb positions. This ended the Bosnian war and the U.S. brokered a peace agreement.

During the 1992-95 war, the United Nations declared Srebrenica a safe haven for civilians. But on July 11, 1995, Serb troops overran the Muslim enclave. Some 15,000 men tried to flee through the woods toward government-held territory while others joined the town's women and children in seeking refuge at the base of the Dutch U.N. troops.

The outnumbered Dutch troops could only watch as Serb soldiers rounded up about 2,000 men for killing and later hunted down and killed another 6,000 men in the woods.

The United Nations admitted its failure to protect the town's people and on Saturday, Bert Koenders, foreign minister of Netherland said that "the Dutch government shares responsibility" and that the U.N. must strengthen United Nations missions in the future.

"Nobody can undo what happened here but we mourn with you," Koenders added.

The 1992-95 war in Bosnia, pitting Christian Orthodox Serbs against Bosnian Muslims and Croatian Catholics, left more than 100,000 people dead and millions homeless. The Serbs, who wanted to remain in the Serb-led Yugoslavia, fought against the secession of Bosnia and Croatia from the former federation.

So far, remains of some 7,000 victims have been excavated from 93 graves or collected from 314 surface locations and identified through DNA technology.

A group of Bosnian Muslims, refugees from Srebrenica, walk to be transported from the eastern Bosnian village of Potocari to Muslim-held Kladanj, July 13, 1995. (Reuters)

A woman searches for a relative's name on the coffins of 136 newly identified victims of the 1995 Srebrenica massacre lined up for a joint burial in Potocari, Bosnia and Herzegovina, July 9, 2015. (Reuters)