Police are reported to have fired tear gas and rubber bullets in an attempt to bring an end to a party which was part of Fête de la Musique in Nantes in the early hours of Saturday, June 21.
The man is thought to have fallen into a river next to the party.
Steve Maia Caniço, 24, was last seen as police moved in to break up the party at 4.30am local time – half an hour after the scheduled end time – with 14 people said to have fallen into the River Loire before being rescued by firefighters.
Mr Caniço’s friends have said he couldn’t swim, while witnesses have said they saw a man struggling in the water.
A French police official said France’s police disciplinary body was investigating the event, following footage of riot police armed with batons near the river, which doesn’t have a barrier, appearing online.