Uefa have confirmed they will investigate the incidents which marred Manchester United's Champions League win against Lille.
French police were criticised by fans for being heavy-handed at the Stade Felix-Bollaert, and tear gas was used during the match.
The investigation will also look at possible ticketing problems which led to supporters appearing to be in danger of being crushed against a 10-foot high safety barrier.
A statement from Europe's governing body read: "Uefa has officially opened an investigation into events at the Champions League first knockout round first-leg match between Lille and Manchester United in Lens on Tuesday night."
Lille, meanwhile, have lodged a formal complaint with Uefa after losing in controversial circumstances. The French club, whose players staged a near walkout after a quickly-taken Ryan Giggs free kick gave the visitors a late victory, also hit back at United manager Alex Ferguson.
Lille coach Claude Puel said he did not know what would come from the complaint about the refereeing to European soccer's governing body but told reporters: "I think United really lacked fair play".
Ferguson said the Giggs goal had been played by the book and was disgusted by the home side's reaction to the goal.
"I think Uefa have got to take some action," the Scot said afterwards. "I've never in my life seen anything like that. It's a disgrace, what we've seen tonight. That's not football. The Lille staff encouraged their players to come off the pitch. It's intimidation of the referee and it's made for a hostile atmosphere inside the ground."
Puel dismissed the remarks, saying: "Alex Ferguson thinks what he wants. But I do not understand that this man indulges in such comments. He could spare them. These comments are just one of Alex Ferguson's usual tricks."
Giggs's free kick caught the whole Lille team off guard but Ferguson said that was part of the game.
"The referee gave the permission to take the kick so the goal is perfectly valid," said Ferguson. "We've already seen this in England. If players ask the referee to take the free kick quickly, they are allowed to do so. Rules are crystal clear."
