Marseille prosecutor Brice Robin has said 150 specially-trained Russian hooligans were behind much of the violence that broke out in the city ahead of the England vs. Russia game on Saturday.
Robin told a news conference that "extremely well trained" people were behind three days of clashes.
"These people were well prepared for ultra-rapid, ultra-violent action," he said. "These are extremely well trained people."
However, Robin said only two Russian nationals had so far been arrested, both for invading the pitch at the England game, and would be expelled from France.
Ten people, six of them British, three French and one Austrian, face immediate court proceedings over the violence. In total, 20 people have been arrested.
Around 35 people have been injured in clashes in the port city.
A 50-year-old England fan is in a critical condition with severe brain injuries after being attacked by Russia supporters armed with iron bars, while clashes also broke out when German hooligans attacked Ukraine fans in Lille.
Robin said "almost all" of the injured were British, but added that England supporters had also been responsible for violence.
On Sunday, the executive committee of European football's governing body warned the FA and the Russian Football Union (RFS) that "it will not hesitate to impose additional sanctions."
In a statement, it said these could include "the potential disqualification of their respective teams" from the tournament in France.
England manager Roy Hodgson and captain Wayne Rooney have recorded video messages to appeal for calm.
A large group of Russian fans in the stands at the Stade Velodrome in Marseille advanced on the neighbouring England section at full-time of the 1-1 draw between the sides on Saturday night, throwing objects and breaking through a line of stewards as England fans fled for the exits.
FA chief executive Martin Glenn said misbehaviour by England supporters in the centre of Marseille before the game had been "embarrassing."
But he stressed that the attacks inside the stadium had been carried out by what he described as "organised Russian gangs."