The policeman was among a group of officers who rushed to the scene after the attacker, who claimed allegiance to the Islamic State group, stormed the store and fired on shoppers and staff before taking them hostage.
The man, identified as 26-year-old Redouane Lakdim, shot dead two people at the supermarket after earlier opening fire on passengers in a car, killing one of them, and injuring a police officer in nearby Carcassonne.
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Speaking at the scene Interior Minister Gerard Collomb said a police team immediately intervened after the start of the siege at around 11 a.m.
"They managed to get some of the people out," he said, but the attacker kept one hostage back as a human shield.
A senior officer then offered to take the hostage's place and remained holed up with Lakdim while negotiations to end the standoff continued.
The officer "left his telephone on the table", to allow police that had surrounded the building in the sleepy town of 5,000 inhabitants listen in, Collomb said.
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"When we heard shots the GIGN (an elite police force) intervened," the minister said, adding that the policeman was "seriously injured".
Collomb praised the officer for his "courage" and "act of heroism".
An officer from the police SWAT team was also hurt in the operation to kill the gunman.
Over the course of his rampage Ladkim killed three people and injured at least three others.
The injured included the driver of the hijacked car as well as a police officer out jogging with colleagues in Carcassonne.
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