French prosecutors have launched an investigation after two police officers were reportedly caught on film making racist remarks about an arrested man.

The incident happened in the early hours of Sunday after a man suspected of theft was pulled out of the Seine river in the Seine-Saint-Denis banlieue north of Paris.

In the video, reportedly made by a local resident, a police officer can be heard using an extremely offensive word for a North African, before adding: “They can’t swim. You should have attached a ball and chain to his ankle.”

The man is then handcuffed and put into the back of a police van, where he can be heard shouting.

The film, posted on Twitter by Taha Bouhafs, a journalist and anti-racism activist, prompted the French interior minister, Christophe Castaner, to tweet: “A video showing a police intervention around the Île-Saint-Denis has provoked legitimate indignation. Light will be shed (on the incident).”

Castaner said the Inspection Générale de la Police Nationale (IGPN) – the police force’s internal police – had been instructed. “Racism has no place in the Republican police force,” he added.

Catherine Denis, a public prosecutor, said the man had been taken to hospital after falling in the Seine and that an investigation was being launched.

“We have to carry out a precise analysis of the video to know exactly what is in it, but we are opening a legal inquiry because the words we hear on this video, if indeed spoken by the police officers, are not acceptable and show an unprofessional attitude that risks discrediting the actions of the police,” Denis told FranceInfo.

Éric Coquerel, an MP representing the Seine-Saint-Denis department, said the police officers’ behaviour was “unacceptable”.

“I don’t know what the person had done … To hear racist words and behaviour from a police officer in this day and age is shameful and extremely worrying,” he said.

The local police prefecture said an internal police investigation would be carried out into “the circumstances in which the officers intervened and to determine the identity of those who were speaking”.

The Paris police chief, Didier Lallement, has demanded the immediate suspension of the two officers alleged to have made racist remarks.

The incident will not soothe tensions in the Paris banlieue, where relations between local youths and police are described by authorities and associations as “complicated”.

Unrest erupted in Seine-Saint-Denis a week ago after an incident in which a motorcyclist hit a police car, leading to clashes between youths – who set fire to rubbish bins and cars, and launched fireworks – and the police, who have been accused of a heavy-handed approach to the lockdown.

The Seine-Saint-Denis department, the poorest in France, has been one of the worst hit by the coronavirus.