Revellers jumped into the Bosphorus river to escape as an attacker, reportedly dressed as Santa Claus, opened fire.

At least 35 people have been killed after a gunman reportedly dressed as Santa Claus attacked a nightclub in Turkey.

The governor of Istanbul has described the incident, which also injured at least 40 others, as a "terrorist attack".

It is believed the gunman shot at police officers outside the Rezia nightclub before opening fire at random inside the building.

Estimates suggest about 500 to 600 people were celebrating the New Year at the venue.

Many people jumped into the Bosphorus river in panic.

Television footage showed ambulances and police vehicles on the street outside the nightclub in the Ortakoy district. 

"A terrorist with a long-range weapon ... brutally and savagely carried out this incident by firing bullets on innocent people who were there solely to celebrate the New Year and have fun," Governor Vasip Sahin told reporters at the scene.

Media reports said police have cordoned off the area and an operation is ongoing.

Security measures had been heightened in major Turkish cities, with police barring traffic leading up to key city squares.

In Istanbul, 17,000 police officers were put on duty, some camouflaged as Santa Claus and others as street vendors, Anadolu reported. 

Ankara and Istanbul have been targeted by several attacks in 2016 carried out by the Islamic State group or Kurdish rebels, killing more than 180 people.

Source: Skynews