A vehicle full of explosives was detonated as military buses were passing by, according to the Ankara governor's office.
The blast happened in an area close to parliament and Turkey's military headquarters.
Deputy Prime Minister Bekir Bozdag called it an "act of terrorism".
Large plumes of smoke were seen rising from the area and witnesses said the blast was heard all over the city.
Some of the victims were civilians.
No group has so far claimed responsibility for the attack.
(BBC News)