The officer told French media how he and a colleague came face-to-face with one of three gunmen launching an attack, and shot him dead.
Ninety concert-goers were killed at the Bataclan concert hall, where Eagles of Death Metal were playing.
Dozens more people died in co-ordinated attacks across Paris on 13 November.
The policeman, a department head at the Paris anti-crime brigade (BAC), told FranceInfo radio (in French) that he and his colleague had been responding to reports of an explosion at the Stade de France when they were redirected.
They were ordered to go to the 10th and 11th arrondissements of Paris following news of shootings. On the way, they found panicked concert-goers fleeing the Bataclan and quickly decided to go inside.
"What surprised us immediately was the extremely bright light that blinded us," said the officer, whose identity has been protected for security reasons.
"The bewildering silence... and then hundreds of bodies one on top of the other."
What happened at the Bataclan?
Who were the victims?
Soon after entering the music venue, the police duo saw a gunman walk on stage and point a Kalashnikov at an audience member.
"He was very calm," the officer said.
"Given the carnage, there was no doubt about what we had to do. We opened fire straight away. We kept shooting until he fell to the floor."
The gunman was later identified as Foued Mohamed-Aggad, a 23-year-old French national from Strasbourg.
The two officers, who reportedly only had service weapons and light bulletproof vests, then heard an explosion from elsewhere in the building and realised there were likely to be more attackers. Faced with extreme danger, the men phoned their loved-ones.
"We decided we could not leave the scene - we could not leave these people," the officer said.
"I was ready to die with my team that night. I phoned my girlfriend to say goodbye."
Elite police units soon arrived at the scene. Two other attackers who took people hostage in the concert hall eventually died in a final police assault hours later.
They were later identified as Frenchmen Omar Ismail Mostefai, 29, and Samy Amimour, 28.
Paris attacks: Who were the attackers?
The first police officer to arrive at the Bataclan has described the "indescribable moment of fear" he felt on entering the scene of the massacre.