Police have been forced to put reinforcements on the capital's overground RER train service after a 20-strong gang carried out an audacious robbery on passengers, while they were stopped at a station.

The  robbers, reportedly armed with pepper spray stormed a train at Grigny station on the RER line D on Saturday. They threatened passengers before relieving them of their personal valuables, in what witnesses described was a “highly organized” attack.

Police have boosted numbers on the line and at Grigny station to reassure passengers.

“Before, these kinds of robberies were generally carried out by four or five people working together, but in this case they numbered 20, which is worrying,” one officer told France’s 20 Minutes website.

France’s Interior Minister Manuel Valls visited Grigny on Tuesday and made a public appeal for both victims and witnesses of the crime to come forward.

The RER service, which links Paris to the surrounding suburbs and Ile-de-France region, has a reputation for being a crime hotspot, which was backed up by figures published on Wednesday.

According to data published in Le Figaro, there are 150 reported crimes each day on the rail network in and around Paris. In total there are around 55,000 crimes each year on the RER and metro services in the French capital, which represents a rise of 1.7 percent on last year.

Although Le Figaro reports that acts of violence on trains decreased in 2012, attacks against members of staff soared by 25 percent.

Opposition UMP deputy Valerie Pécresse wants more CCTV cameras installed on RER trains and at stations, to try and cut down crime.

“This attack marks a change in the methods used by these gangs,” Pécresse said. “CCTV needs to be used in real time by the police and there needs to be more security staff in place,” she said.

Jean Claude Delarue from rail users group FUTSP wants the government to do more to protect travellers on the RER.

“We are asking the minister to make decisions and not just send in a few extra policemen for a couple of days,” said Delarue.

In February this year, a gang of similarly audacious robbers held up a Paris-bound train in Marseille, in what one policeman described as like a "scene out of  the wild west". Luckily they were unable to board the train.