French security forces were facing embarrassment as leaked court documents revealed police initially concealed information regarding the circumstances of the death of a young environmental activist.

Remi Fraisse was fatally wounded during clashes between demonstrators and police at the site of a controversial dam project in southwestern France the night between 25 and 26 October.

Days later, an autopsy revealed that the 21-year-old died instantly, after a concussion grenade hit him in the back.

The morning after the clashes, local authorities said they had found the body of a protester, adding it was "impossible" to immediately determine the cause of death.

However, the transcript of taped conversations between riot police officers involved in the violence at the Sivens dam project in woodlands northeast of Toulouse suggests they knew what had happened all along.

Minutes before 2am that night, a gendarme ordered colleagues to stop firing concussion grenades after witnessing a man - Fraisse - falling onto the ground, according to the document seen by Le Monde.

"It's ok, he is going to get back on his feet, it's ok," a policeman says.

About 10 minutes later, seeing he is still on the ground the officers approach Fraisse.

"Is he breathing or what?" one says as a medic tries to revive him.

"The bloke is dead ... There, it's very serious ... they must not know," another one says a couple of minutes later.

A police spokesperson told the newspaper that the sentence didn't connote the intention to cover up Fraisse's death. The policeman was worried that if the information was immediately passed onto other protesters it could have fuelled more violence, according to the spokesperson.

The revelations on the case, which is being investigated by local prosecutors, prompted calls for interior minister Bernard Cazeneuve - who was severely criticised for remaining silent on the incident for 48 hours - to resign.

"Cazeneuve knew. He took two days. He lied. He has to resign," leftist MEP Jean-Luc Mélenchon tweeted.