Dozens of people were killed during New Year's celebrations when at least one gunman opened fire on revellers in one of Istanbul's most popular nightclubs, in the latest in a series of deadly terrorist attacks that have shaken Turkey.

Istanbul's Mayor, Vasip Sahin, put the death toll at 35, with at least 40 people wounded.

Witnesses described gunmen spraying the crowds with gunfire, and stampedes of panicked patrons scrambling for cover at the waterside club, called Reina.

The assault began about 1 am local time, when a gunman shot and killed a police officer who was guarding the door, according to Sahin, who spoke to reporters in front of the club about two hours after the shooting.

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After killing the policeman, the gunman "brutally and violently attacked innocent people who came here to enjoy themselves," Sahin said, calling it a "terrorist attack". Sahin mentioned only one assailant, but other witnesses, speaking to local media, said they had seen two or more.

The attacks have come despite a widening security dragnet by the authorities, and a broad crackdown by the Government on enemies officials have branded as terrorists. Security measures had been heightened in major Turkish cities, with police barring traffic leading up to key squares in Istanbul and the capital, Ankara. In Istanbul, 17,000 police officers were on duty, some camouflaged as street vendors.

There was no immediate claim of responsibility for the shooting. The mass killing at the nightclub was at least the fourth major attack in Turkey in less than a month, raising questions about the ability of the Government, a Nato member and critical regional ally of the United States, to counter an array of threats stemming from the war across Turkey's border in Syria, as well as an escalating conflict with Kurdish militants inside Turkey.

At least one of the recent assaults - a suicide bombing at a football stadium in central Istanbul - was claimed by a Kurdish militant group. But the authorities are still investigating who might have planned other attacks, including the assassination last week of Russia's Ambassador to Turkey, by a police officer who condemned the carnage in Syria's civil war as he gunned down the envoy.

Medics carry a wounded person at the scene after an attack at a popular nightclub in Istanbul. Photo / AP
Medics carry a wounded person at the scene after an attack at a popular nightclub in Istanbul. Photo / AP

Turkey has recently taken a central role in trying to halt the hostilities in Syria, in coordination with Russia, which is allied with the Government of Syrian President Bashar al-Assad. A recent ceasefire announced by Turkey and Russia was endorsed by several Syrian rebel groups, but it did not include others, including extremist Islamist groups that might seek to retaliate against the Government of Turkish President Recep Tayyip Erdogan because of its cooperation with Russia.

At the time of the attack, there were hundreds of people inside the club, which sits on the Bosporus strait, according to local media. There were unconfirmed reports that the gunman stormed the club wearing a Santa Claus costume. Patrons jumped into the water to escape the gunfire, according to social media reports, and dozens of ambulances could be heard heading in the direction of the club, in Istanbul's Ortakoy district.

The club's owner, Mehmet Kocarslan, told the Hurriyet that there had been increased security at the club for a week or more, because of warnings from "American intelligence". He told the paper that the gunmen had used Kalashnikov rifles.

"We were there, we were having fun, when all of a sudden people started running," Sinem Uyanik, a patron of the club, told Hurriyet. Uyanik was waiting outside a hospital, where her husband, who was also at the club, was being treated for gunshot wounds.

"Before I could understand what was happening, my husband fell on top me. I had to lift several bodies from on top of me before I could get out."

Uyanik said she saw two gunmen at the club.

"I didn't see who was shooting but heard the gun shots and people fled. Police moved in quickly," Sefa Boydas, a football player, tweeted.

Security forces later stormed the nightclub, Uyanik said. Police in riot gear and armoured vehicles blocked the area around the venue.

Youths embrace near the scene of an attack in Istanbul. Photo / AP
Youths embrace near the scene of an attack in Istanbul. Photo / AP