Emergency workers work at the explosion site in Ankara, Turkey March 13, 2016. [Photo/Agencies]

One of the security officials said the car used in the attack was a BMW driven from Viransehir, a town in the largely Kurdish southeast, and that the PKK and the affiliated Kurdistan Freedom Hawks (TAK) appeared to be responsible.

TAK claimed responsibility for the previous car bombing, just a few blocks away, on Feb. 17. That attack targeted a military bus as it waited at traffic lights, and killed 29 people, most of them soldiers, near the military headquarters, parliament and other key government institutions.

A police source said there appeared to have been two attackers, one a man and the other a woman, whose severed hand was found 300 metres from the blast site.

The explosives were the same kind as those used on Feb. 17 and the bomb had been reinforced with pellets and nails to cause maximum damage, the source told Reuters.

The pro-Kurdish opposition HDP, parliament's third largest party, which Erdogan accuses of being an extension of the PKK, condemned what it described as a "savage attack".

State broadcaster TRT said the car had exploded at a major transport hub, hitting a bus carrying some 20 people near the central Guven Park and Kizilay Square at 6:43 pm (1643 GMT).

An Ankara court ordered a ban on access to Facebook, Twitter and other sites in Turkey after images from the bombing were shared on social media, broadcasters CNN Turk and NTV reported.