Russian hooligans who attacked England fans in Marseille were "trained to fight", according to the city's chief prosecutor.
Marseille was rocked by three days of violence in the lead up and aftermath of England's Euro 2016 clash with Russia, which took place on Saturday evening.
Much of the unrest now appears to have been instigated by Russian hooligans.
"There were 150 Russian supporters who in reality were hooligans," Brice Robin said.
"These people were well prepared for ultra-rapid, ultra-violent action. These are extremely well-trained people."
Robin said some Russian supporters were turned back on arrival at Marseille's international airport but that others had arrived overland.
Most of the 35 people who were injured in the fighting were English, the prosecutor said. Three people were seriously hurt and one England fan remains in a "critical but stable" condition.
"His attackers have not been identified," Robin said.
Robin added that two Russian nationals were being expelled from the country.
Of the 20 people arrested, six Britons, three French and one Austrian supporter will appear in court on Monday.
Authorities in the Lyon area announced a ban on sales of alcohol to football fans when Euro 2016 matches are played in the city.
However, the ban will not apply to a fan zone in the city centre where beer is consumed on site.
Lyon will host its first match on Monday, pitting Belgium against Italy with 15,000 supporters expected on each side.
Interior Minister Bernard Cazeneuve had on Sunday asked all cities hosting Euro 2016 matches to take "all necessary measures... to prohibit the sale, consumption and transport of alcoholic drinks in sensitive areas on match days and the day before, and on days when fan zones are open."