At least 39 people die in an attack on a New Year's Eve party at the Reina nightclub in Istanbul
It happened at about 01:30 local time (22:30 GMT)
The city was on high alert with about 17,000 police officers on duty
Turkey's interior minister says police are still searching for the assailant
Gunman left his weapon at the scene, Turkish PM says
Image source, Getty ImagesAs day broke, forensic officers were at work inside the waterfront venue
Image source, Getty ImagesPolice boats patrolled the waters outside the venue. The gunman, who killed 39 people, is still at large.
Image source, Getty ImagesArmed police were on patrol on the streets of Istanbul
In the US, the White House condemned the "horrific terrorist attack".
A spokesman said President Obama, on holiday with his family, had been briefed on the attack.
The US sent "thoughts and prayers" to the relatives of those killed.
Pakistan's Prime Minister Nawaz Sharif said his country shared Turkish people's grief and stood by them in a "painful" time.
"Terrorism is our common enemy and the world needs to put up a joint fight against this menace," he said.
Turkey's leading cleric Mehmet Gormez condemned the gun attack on "vulnerable people" which "hurt our nation deeply".
"There is no difference whether this inhuman massacre is carried out at a bazaar, a place of entertainment or a place of worship," Gormez, head of the Religious Affairs Directorate, said in a written statement
"Terrorists do not target places, but people, nations and the whole humanity. Terrorism cannot be tolerated whoever or whatever it comes from," he said.
Turkish authorities have given only a very small amount of information so far, BBC Turkish reporter Rengin Arslan said.
There is a media ban on reporting on the attack.
And there are there are conflicting reports about the outfit of the attacker.
Initial reports and eyewitnesses accounts suggested he had a Santa Claus costume but newly obtained CCTV footage shows him in a black coat outside the night club.
The attacker is still at large and whole city is on alert to try to find them, BBC Turkish's Rengin Arslan said.
There is no official statement about who might be behind the brutal attack, but the finger of blame is pointing towards so-called Islamic State.
Over the past two years there have been a spate of attacks in Turkey.
Kurdish militants have mostly targeted military forces and police; IS has targeted civilians.
IS leaders have threatened Turkey and called on their followers to carry out attacks against the country.
Turkey started a ground operation against IS, as well as Kurdish groups inside Syria, four months ago.
The Turkish government said the assailant is still at large after what it described as a "terrorist attack".
Initial reports said he had been wearing a Santa Claus suit. Unconfirmed video footage has emerged showing a gunman in a white outfit.
Some reports said there was more than one gunman and that party-goers jumped into the Bosphorus, alongside the waterfront club, to escape the attacker.
One Turkey attack survivor describes the scene. There is no confirmation that there was more than one attacker
We will be bringing you updates on the attack on a nightclub in Istanbul: