Dozens of "highly trained" Russian hooligans who descended on Marseille ahead of their country's European Championship match against England were behind much of the worst violence that erupted before, during and after the match, a prosecutor said Monday.

Meanwhile, a riot police union spokesman, Dominique Mesquida, said law enforcement officers knew it was a high-risk match, but were not specifically warned to be on the lookout for Russian thugs.

"We weren't told, 'Be very careful, you're going to face ultra-violent Russian groups,'" said Mesquida, a representative of the Unite SGP Police-FO union for the CRS riot police in the Marseille region.

The comments painted a worrying picture of well-organized, violent Russian hooligans roaming the streets and deliberately attacking England fans as riot police were almost powerless to stop them. Russia's next match is Wednesday in the northern city of Lille, only 30 kilometers (20 miles) from Lens, where England plays the following day.

Marseille prosecutor Brice Robin told reporters that about 35 people were injured in three days of clashes around Saturday's match that ended in a 1-1 draw at the Stade Velodrome, and that one England fan remained in a critical but stable condition. Brice said almost all the injured were British.

Twenty people were arrested by police who struggled to control running street battles between fans in the city's Old Port and on roads leading to the stadium. Ten suspects - six Britons, an Austrian and three French - will face immediate trial at a courthouse in Marseille.

Two Russians were among those arrested, but none was among the suspected hooligans facing swift justice, Robin said. He said about 150 Russian hooligans evaded attempts to keep them out of France during the month-long tournament, though some others were stopped at Marseille's airport and expelled from France.

The Russian hooligans "were prepared for hyper-quick and hyper-violent intervention, and that's where the difficulty came from in proceeding with their arrests," Robin said. "I will not say they are violence professionals, but they were highly trained."

An Associated Press reporter followed one group as it moved swiftly along a broad tree-lined avenue leading to the stadium ahead of Saturday's match followed by plain-clothed police who regularly fired tear gas, but failing to halt their progress. Once the hooligans arrived at a roundabout outside the stadium they immediately charged into British fans and then dispersed as police again fired tear gas and used a water cannon.

Mesquida described the Russians as "paramilitary groups" and said they arrived in Marseille, "with the intention to hurt, to cause injuries among the British."