A gunman has killed dozens of people inside a nightclub in Istanbul during New Year's Eve celebrations.
At least 35 people have been killed and another at least 40 people have been injured in the attack, according to the Istanbul governor.
Large numbers of police and paramedics were sent to the well-known Reina bar in the Ortaköy district, CNN Turk reported. The attack happened in the very early hours of 2017, targeting a New Year's Eve party at the popular club.
The attackers are believed to have shot at police outside the nightclub before entering and firing on people inside using a long-barrelled weapon, according to the city's governer Vasip Sahin, who described the event as a "terror attack" but did not name who he thought responsible. "Unfortunately (he) rained bullets in a very cruel and merciless way on innocent people who were there to celebrate New Year's and have fun," Mr Sahin said.
Some customers jumped into the waters of the Bosporus to escape the attack, reports claimed.
At least six ambulances were outside the club transferring the injured to hospital for treatment. As many as 600 people are thought to have been in the club when the attacker entered it, not longer after midnight.
At least one armed man was involved in the attack, reported NTV, and police launched an operation to capture an assailant who was said to have been in the club after initial reports. He may have been dressed in a "Santa costume".
There was no immediate confirmation of casualties from police.
Reina is a large and well-known club in the middle of Istanbul.
Much of Turkey had been subject to increased security measures on New Year's Eve, after a year that has seen deadly attacks by Isis and Kurish extremist groups.
Traffic leading up to key squares in Istanbul and the capital, Ankara, were closed, police had said. Around 17,000 officers were on duty in the city, some of them disguised as Santa Claus and as street vendors, according to the Anadolu news agency.
Twin bombings carried out by the Kurdistan Freedom Falcons (TAK) killed 44 people on 10 December, while other blasts have targeted police and the security services.
Isis was behind the attack on Istanbul Airport in July, as well as previous suicide bombings targeting tourists and shoppers.
A supporter of the so-called Islamic State was responsible for massacring 49 people at an LGBT-friendly nightclub in Orlando, Florida, in June.
