The Gare du Nord is a busy commuter hub which also serves as a departure point for trains to northern France, London and northern Europe.
Police were treating the stabbings as attempted murder, not as a terrorist attack, a source close to the case said. The attacker’s motive was not immediately clear.
One of the six people was seriously wounded in the attack at 6:42am (5:42am GMT) while the other five sustained light injuries, prosecutors said.
The entire incident was over within two minutes, Interior Minister Gerald Darmanin told reporters at the scene.
It was not immediately clear what type of weapon the attacker had used. Police initially described it as a bladed weapon. Darmanin later said it was not actually a knife, but a “threatening weapon”.
Le Parisien newspaper said it was believed to be a sharp pointed tool, “a kind of awl”.
The attacker sustained injuries to the chest after police fired three rounds. He was taken to hospital where he was fighting for his life, Darmanin said.
The police who intervened were off-duty officers in plain clothes who used their service weapons to stop the attack, he said.
The police officers “were returning from duty at police headquarters to take their train and go home, but they were armed”, Darmanin said.
They were authorised to make use of their weapons although not on duty, he said. One member of the border police deployed at the station was lightly injured.
“An individual injured several people this morning at the Gare du Nord,” Darmanin had tweeted earlier.
“He was quickly neutralised. Thank you to the police for their effective and courageous response”.
The Gare du Nord is one of the world’s busiest train stations with 700,000 travellers per day. It is the departure point for Eurostar and Thalys services to the UK, Belgium, the Netherlands and Germany.
France remains on a state of heightened security alert after a spate of deadly attacks by Islamist radicals and others since 2015.
The country has repeatedly taken actions and put restrictions in an attempt to fight what it describes as “Islamist separatism”.