The gunmen -- one of whom was thought to be a Primark employee -- were still on the loose and believed to be inside the Qwartz mall in Villeneuve-la-Garenne just north of Paris.
"Around 6:30 am (0430 GMT), three armed criminals went into the Primark store for what we think was initially an attempt at armed robbery," said a police source, who wished to remain anonymous.
"But we don't know whether they are still there."
An employee first alerted her boyfriend of the hold-up at around 7:00 am when she sent him a text message saying they had been taken hostage by two armed men, another source said.
The area around the mall was shut down, all traffic stopped and other stores in the vicinity closed up as special forces rushed to the scene.
The mall was promptly evacuated but according to police, some 18 employees had remained inside, holed up in a canteen.
It is as yet unclear whether they were taken there by the gunmen or whether they escaped on their own and hid there.
France on high alert
They "were evacuated around 10:30 am, one of them was unwell but no-one was injured," police said, adding security forces were still hunting for the gunmen.
The armed men were caught entering the mall on CCTV cameras but have not been filmed leaving the premises, which indicates they are still inside.
The memory of attacks in January by Islamic extremists who gunned down 17 people in and near Paris -- including several who were held hostage in a Jewish supermarket -- is still fresh in the minds of the French.
France, and the Paris area in particular, is still on high alert and some 7,000 soldiers are deployed around the country to help police in countering any further attack.
But so far police have indicated that Monday's attack appears to be an attempt at armed robbery.
According to a source close to the probe, one of the three suspects was recognised as being a Primark employee.
A London-based spokesman for Primark, a low-cost clothing store that has been a huge success in Britain, confirmed there had been an "incident" at its store just north of Paris.
"Police are in attendance... We will do all we can to support our staff and their families," he said.
"We hope for a speedy and safe end to this incident."
The incident comes on the eve of the traditional military parade that takes place every year in Paris on July 14, France's national day.
This year, special forces such as the ones that intervened Monday will march down the Champs-Elysees for the first time in recognition of the role they played in countering the January attacks.