A group of "well-trained" Russian hooligans were behind the violence in Marseille when Russia played England in a Euro 2016 match, prosecutors say.
The 150 Russian football supporters "were well prepared for ultra-rapid, ultra-violent action" and were able to evade arrest, a city prosecutor said.
Five England fans have been jailed for throwing bottles at police and a sixth jailed in connection with the violence.
Two Russians have been arrested, both for a pitch invasion.
Some 35 people have been injured - four seriously, and most of them England fans - and a total of 20 people have been arrested after three days of disorder in the French city.
Home Secretary Theresa May accused Russian football football fans of bearing a "heavy responsibility for initiating violence", but said some England fans had let their country down.
Meanwhile, England manager Roy Hodgson and captain Wayne Rooney appealed directly to fans to "behave themselves".
The clashes in Marseille's Stade Velodrome on Saturday followed England's 1-1 Euro 2016 draw with Russia, after Russian fans appeared to rush at England supporters.
Marseille prosecutor Brice Robin told a news conference: "There were 150 Russian supporters who in reality were hooligans. These people were well prepared for ultra-rapid, ultra-violent action. These are extremely well-trained people."
This was what made it difficult to arrest them, the prosecutor said, adding that he did not think the Russian hooligans were "professional" but that they were "extreme".
He was speaking as six England fans appeared in court for sentencing, following the disorder.
The first fan to appear in court in Marseille on Monday was 20-year-old Alexander Booth, from Huddersfield, West Yorkshire, who was sentenced to two months for throwing a plastic bottle at police.
Reacting to the sentence, Booth's father Chris, who was later seen in tears, said he would "fight it".
Iain Hepworth, 41, a psychiatric nurse from Sheffield, was sentenced to three months for throwing a bottle at police and then walking forward to pick up a second bottle.
Ashley Kelly, 26, from Birmingham, was sentenced to three months in prison for throwing seven bottles outside the stadium.
Both Paul Jackson, 21, from Halifax, and Lee Phillips, 23, from Taunton, were given one month in prison.
Steven Cornell, 28, was also jailed for two months after being charged in connection with the violence.
All six have been banned from visiting France for two years each.
In other developments:
- Another four people - three French and an Austrian - are due to appear in court imminently
- A 16-year-old Briton was among those arrested and charged with throwing bottles
- Two Russian nationals were being expelled from the country, French prosecutors said
- Seven England fans are still in hospital, Mrs May told the Commons
- An England fan, who is in an induced coma with severe brain injuries, has been named as Portsmouth supporter Andrew Bache, 50, known as Pepe
- British officers are being sent to Lens ahead of the England-Wales match on Thursday
- The French government urged cities hosting Euro 2016 matches to ban alcohol near venues
- Lens, in northern France, banned alcohol from being sold
During the press conference, Mr Robin said officers were looking at pictures with foreign colleagues - Russian and English football spotters - to try to identify those involved in the disorder.
And British Assistant Chief Constable Mark Roberts, the National Police Chiefs' Council lead for football policing, appealed for fans with their own camera phone footage to contact crime-fighting charity Crimestoppers.
Witnesses said trouble began in the stadium on Saturday after flares were let off by Russian fans near the end of the game. Some then climbed across barriers designed to keep rival fans apart.
A number of Russia supporters appeared to kick and punch fleeing England fans, who were forced to clamber over fencing to escape.
Uefa has been investigating and has threatened to ban both teams if there is further violence.
It also opened disciplinary proceedings against the Russian Football Union for alleged crowd disturbances, racist behaviour and the setting off of fireworks.
There were 12,000 Russian fans in Marseille who travelled to the city by train, according to French prosecutors.
Sanctions against Russia will be decided at a disciplinary meeting on Tuesday, Uefa said.
Russia's sports minister, Vitaly Mutko, said Uefa had "done the right thing" to start a disciplinary case against the Russian Football Union, R-Sport news agency reported.
But Russian MP Igor Lebedev criticised French police and called on Russian football fans to "keep up the good work".
Russia's next match against Slovakia will take place in Lille on Wednesday - the day before England's match against Wales, just 24 miles away in Lens.
In a separate incident not related to the disorder in Marseille, a Northern Ireland football fan died after falling from a promenade in Nice following the team's 1-0 defeat by Poland.