French prosecutors said Monday that around 150 Russians were behind most of the violence that broke out in Marseille before the England vs. Russia match in the ongoing Euro 2016 soccer tournament. Twenty people have been arrested so far, of which 10 face an immediate trial.

The violence during Saturday’s game left at least 35 people injured, four of whom were reportedly in a serious condition. “These people [the Russians] were well prepared for ultra-rapid, ultra-violent action,” Marseille prosecutor Brice Robin said at a news conference. “These are extremely well trained people.”

He added that of the 20 people arrested, only two were Russians. A 16-year-old Briton, charged with throwing bottles during the clashes, was also arrested. Reports said two other Russians have been expelled from France.

The 10 people facing immediate trial include six Britons, three French and an Austrian. Robin said that the majority of those injured were Britons.

According to the Independent, Russian fans were seen carrying gum shields, fighting equipment and mixed martial arts gloves. A 50-year-old Briton is in critical but stable condition after being attacked by Russian supporters armed with iron bars; he reportedly suffered a concussion and brain hemorrhage.

Robin added that about 12,000 Russians travelled to Marseille by train. He said he did not think the Russian hooligans were “professional” but they “were extreme.”

Earlier on Monday, France banned the sale and consumption of alcohol in match venues and fan zones following the violence that erupted over the weekend. The U.K. reportedly offered to send British police officers to help local security at Lens where England takes on Wales Thursday.