The blaze broke out at the height of the party, soon after midnight. As a local DJ played, a guest or a staff member stumbled while carrying the birthday cake with 20 lit candles and sparklers down steep wooden stairs into the basement.
The cake went flying and the candles hit the walls, which witnesses said were covered with a soundproof material that caught fire immediately.
The basement, packed with about 20 people, rapidly filled with thick black smoke and toxic fumes.
"I was on the ground floor of the bar. We saw the flames, it was like a flame-thrower," said a customer, Stéphanie, 31.
Amar Ould Said, 40, a witness, said: "They were bringing bodies out every two minutes. They set up a first aid area on the pavement across the road."
The blaze was so intense that it took 80 firemen half an hour to bring it under control. About 40 police officers also rushed to the scene.
Some of the dead were so badly burned that they remained unidentified on Saturday night. Frantic relatives rushed to Rouen hospital where psychologists counselled the bereaved. At least 50 people were searching for missing friends or family.
The horrific fire plunged Rouen into profound shock for the second time in 10 days after Islamist terrorists slit the throat of an 85-year-old priest in Saint-Etienne-du-Rouvray, only six miles from the bar in the town centre.
Initial reports of the fire raised fears of another attack, but the mayor, Yvon Robert, said it was "completely accidental".
The basement where partygoers were trapped reportedly had "no proper fire exit", according to regular customers of the bar, known as a music venue that stayed open until 2am.
It was popular with people from Réunion, an Indian Ocean island that is part of France. Rouen is home to a sizeable Réunionnais community and one guest, Willy, 20, said: "This bar was more like a family than a bar."
But another regular customer complained about the lack of safety.
"It's a real mousetrap," he told Le Parisien newspaper. "Could you see an illuminated sign, an exit route or a way out? Nothing like that. In the panic, how would you get out?"
There was a door leading from the basement to a corridor and garages behind the bar, but the guests appear not to have managed to exit through it, although firemen used it to gain access to the basement.
Questions were also asked about whether the stairs, described by a regular customer as "almost like a ladder", complied with safety standards. Colonel Pascal Belhache, a fire expert, said: "The way the materials reportedly caught fire so fast raises doubts about whether they were approved under fire regulations."
Forensic teams on Saturday examined the charred remains of the bar, whose owner, named as Nasser, was unhurt.
President Hollande expressed his "emotion and sadness" and promised: "Everything will be done in the judicial investigation in progress to determine the causes of this dramatic accident."
A friend of the policewoman, also called Ophélie, described her as "a young woman who was always happy, generous, passionate about her job and also about horses."
