Guests at an event organised by Russia’s leading human rights group, Memorial, have been attacked by nationalist activists, the organisation’s executive director has said.

Participants arriving at the award ceremony for young history students were pelted with green disinfectant and ammonia, said Yelena Zhemkova.

“Memorial was holding a very important event at Dom Kino in central Moscow, but the guests and the participants were attacked by a group of aggressive protesters,” Zhemkova said.

The protest was organised by the People’s Liberation Front nationalist movement, local media reported. Roughly 20 activists are said to have congregated outside, holding banners reading “we don’t need alternative history” and shouting insults.

Among those attacked was acclaimed Russian novelist Lyudmila Ulitskaya. The writer, who headed the jury of the school competition, was sprayed in the face with green disinfectant.

A number of international guests were also present, including the German ambassador to Russia, Rüdiger von Fritsch, Novaya Gazeta newspaper reported.

Школьный конкурс Мемориала, при входе всех встречают воплями "национал-предатели" pic.twitter.com/fsDtafu5si — Memorial (@MemorialMoscow) April 28, 2016

The nationalist group’s youth wing coordinator Maria Katasonova denied the attack on Ulitskaya in an interview with the Govorit Moskva radio station. “We don’t know who sprayed Ulitskaya,” she said. “I only saw her turn around and she was already covered in green disinfectant.”

The school competition, titled “A person in history: Russia in the XX century” is an annual event organised by Memorial inviting students from around the country to research local history by studying archives, interviewing locals and examining newspapers and other sources.

Winning students are then invited to Moscow where they attend a number of events organised by Memorial. The culmination of their Moscow programme is the awards ceremony.

Police arriving on the scene after complaints of a disturbance said that the protest was a one-man picket and took no further action.

“Usually, even it’s a real one-man protest, the police will come and put everybody in the back of a van. This time nothing happened, even though our colleague had an eye injury,” Zhemkova said.

The executive director said that although there had been previously protests carried out during previous Memorial events, it was the first time activists had been so aggressive.

A protest had been held in front of the Sakharov Centre where Memorial held an exhibition dedicated to the first Chechen war last month, but no one had been attacked, she said.

A version of this article first appeared on The Moscow Times