The attack was aimed at a convoy of military service vehicles, Ankara governor Mehmet Kiliclar said, quoted by the CNN-Turk and NTV channels.
Plumes of smoke were seen rising over an area and the powerful blast was heard all over the city, sending residents to their balconies in panic, an AFP correspondent said.
Ambulances and fire engines were sent to the scene, which is near the Turkish military headquarters and the parliament.
NTV television said the explosion happened near a residential block for top-level military staff.
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Turkish police have thrown a security cordon around the area.
There was no immediate indication about who carried out the attack, but the Islamic State group has been blamed for a string of bombings in the country since the middle of last year.
The capital was already on alert after 103 people were killed on October 10 when two suicide bombers blew themselves up in a crowd of peace activists in Ankara, the bloodiest attack in the country's modern history.
Those attacks were blamed on IS jihadists, as were two other deadly bombings in the country's Kurdish-dominated southeast earlier in the year.
Turkish authorities have in recent weeks detained several suspected IS members, with officials saying they were planning attacks in Istanbul and Ankara.
But Turkey is also waging an all-out assault on the outlawed Kurdistan Workers' Party (PKK) which has staged dozens of deadly attacks against members of the security forces in the southeast.
The PKK launched an insurgency against the Turkish state in 1984, initially fighting for Kurdish independence although now more for greater autonomy and rights for the country's largest ethnic minority.
Ankara has also been carrying out air strikes against Syrian Kurdish fighters across the border wartorn Syria since the weekend.