Footage broadcast on the channel showed police removing mutilated body parts and a bloodied man being taken to an ambulance.
Although NTV television earlier quoted a senior official as saying that an "accident" may have caused the blast, the station later quoted police sources saying the most likely cause was a bomb.
Police were considering the possibility of a suicide attack, CNN-Turk and NTV television reported.
"Police officials are investigating the cause, announcements will be made following the inspections," the Mehmet Ali Sahin, the deputy prime minister, said.
Kurdish separatists, left-wing radicals and hardline Muslims have all launched bomb attacks in Turkey in the past.
In 2003, 30 people were killed and 146 wounded when suicide car bombs hit two synagogues in Istanbul. Five days later, 32 people were killed in similar attacks on the British consulate and HSBC bank in the city. The bombs were blamed on al-Qaeda.
Kurdish fighters launched a series of bomb attacks on tourist sites in Turkey last year, killing more than a dozen people.