The New Year’s countdown had been and gone, but the guests in Istanbul’s lavish Reina nightclub weren’t ready to go home yet.

Hundreds of revellers had come to see in 2017 at the club on the shores of the Bosphorus river, renowned for being a chic party spot for celebrities from Turkey and the rest of the world – people such as Turkish footballer Sefa Boydas who arrived a little after midnight, joined by his girlfriend and her sister.

Once through the doors, the young Beylerbeyi player checked in on the location-sharing app Swarm, before settling into the party atmosphere.

Within 10 minutes, he and the rest of the partygoers were under attack.

A gunman entered the venue and began mercilessly shooting at the crowds. “Just as we were settling down, by the door there was a lot of dust and smoke. Gunshots rang out," Mr Boydas told AFP.

“When those sounds were heard, many girls fainted.”

Mr Boydas recounted how panic quickly ensued as hundreds of people fled, with many being crushed amid the desperate attempts to stay alive. The screams drowned out anything the gunman was saying.

“Even if they [the gunman] were shouting, you wouldn't hear because the crowd's screams were 100 times louder,” he said.

The footballer had to walk over piles of bodies with his girlfriend slung over his shoulder.

“My girlfriend was wearing high heels. I lifted her and carried her out on my back,” he said.

Footage of Istanbul attack and aftermath

“They say 35 to 40 died, but it’s probably more, because when I was walking, people were walking on top of people. You just don't think that this kind of thing would happen in a place like Reina.”

Amid the chaos at least 39 people were shot dead, and scores more were injured.

A police officer outside the venue is thought to have been the killer's first victim.

Mehmet Dag, who watched the catastrophe unfold from the street, said: “The man targeted the security there ... he took most of them down and went inside.

“Once he went in, we don't know what happened. There were gun sounds and after two minutes, the sound of an explosion.”

Sinem Uyanik, who was in the nightclub when the gunman struck, told the Hurriyet newspaper she had been enjoying the night with her husband, when they suddenly found themselves in a stampede around people “soaked in blood”.

“We were having fun. But all of a sudden people started to run,” she said.

“My husband said don't be afraid, and he jumped on me. People ran over me.

Medics carry a wounded person after an attack at a popular nightclub in Istanbul (AP)

“My husband was hit in three places. I managed to push through and get out, it was terrible. People were soaked in blood.”

Maximilien, an Italian tourist, described it as a “night of horror”.

“We were here to have a good time today, but all of a sudden it transformed into chaos and a night of horror,” he told the AFP news agency.

Ismail Celebi, who was across the street from the nightclub, described seeing people “running out, stepping on each other” after hearing a bang.

“As far as we could see, people leaving had big wounds on their bodies. It's hard to believe,” he told the Associated Press.