A reported explosion hit a McDonald's in the French town of Grenoble on Thursday after a large device was spotted in a toilet.

Police closed roads surrounding the fast food restaurant to investigate the cause of the alleged blast, which happened at about 11.50am local time.

Police have not confirmed the 'explosive nature of the event', but a police insider suggested that the blast could have been the result of a big firecracker.

Police closed roads surrounding the McDonald's in the French town of Grenoble on Thursday after a reported bomb went off in one of the restaurant's bathrooms

It is believed that the alleged blast happened after employees on the ground floor saw flames on the second floor and alerted police, according to La Dauphine.

Thirty people had to be evacuated from the restaurant. There are no reported injuries.

The upstairs bathroom at the fast food restaurant was heavily damaged in the incident.

A bomb squad arrived at the scene to investigate the incident. French officials said other McDonald's restaurants in the area will also be investigated.

Authorities have asked people to avoid the area surrounding the McDonald's.

Hours after the incident, Cours Jean-Jaurès, one of the main roads in Grenoble and the location of the McDonald's, was re-opened to drivers.