A spate of muggings early Monday marred a charity concert in South Africa at which Beyonce, Jay-Z and other stars performed in honor of Nelson Mandela.

Tens of thousands flocked to the FNB Stadium in Johannesburg on Sunday for a music festival also featuring Usher, Pharrell Williams, Trevor Noah, Oprah Winfrey, Naomi Campbell and South African President Cyril Ramaphosa. Most people had a good time and got home safely, but the night turned ugly for some when they were accosted by thugs as they left the stadium.

The witness accounts of thieves brazenly assaulting a crowd shocked many South Africans, even in a country with a high rate of violent crime. The muggings tarnished Global Citizen Festival: Mandela 100, whose organizers campaign for girls’ education, an end to extreme poverty and other causes. Many people got tickets to the concert through charity work and petition-signing.

“The criminals were basically having a field day,” said Zikhona Tshona, a reporter for the South African news organization eNCA. She said menacing men made eye contact with some concert-goers, as though to say, ”‘You could be next,’” and that there were no police in sight.

“People’s bags are getting snatched, people are losing their phones, people are fighting in the midst of us looking for an Uber,” a concert-goer said in a video posted on YouTube. He said there was a stampede when someone pulled out a gun.

“The safety of our guests is always of paramount importance to us and we understand” the hurt and anger of those accosted by criminals, Global Citizen, an international advocacy group, said on Twitter. It said it was “saddened to hear the challenges people had while leaving the venue.”

Global Citizen and another group backing the concert, the House of Mandela, urged South African police “to act swiftly and efficiently.”

Police arrested one suspect in the theft of a cell phone, police Brig. Vish Naidoo said.

“We find that there’s a lot of communication taking place on the social media platform, but that is not being reported to us,” said, Naidoo, adding that it is therefore “very difficult for us to deny or even confirm that such things have happened.”

Global Citizen and the House of Mandela are deeply disturbed by the reports of criminal activities against festival guests as they exited the venue on Sunday night. 1/3

— Global Citizen (@GlblCtzn) December 3, 2018

The safety of our guests is always of paramount importance to us and we understand and strongly empathise with their hurt and anger. 2/3

— Global Citizen (@GlblCtzn) December 3, 2018

The South African Police Service has issued a statement requesting all victims of wrongdoing file a police report, which we strongly encourage. 3/3

— Global Citizen (@GlblCtzn) December 3, 2018

Guns N’ Roses performed at FNB Stadium on Thursday. Some World Cup soccer matches were successfully held there when South Africa hosted the tournament in 2010. The stadium hosted a memorial for Mandela after the anti-apartheid leader died in 2013: a sign-language interpreter turned out to be a fake, raising questions about security.

Also in 2013, thieves made off with cash takings at the stadium after a Justin Bieber concert.