The government said Kurdish rebels or Islamic State militants were likely responsible, while mourners accused President Recep Tayyip Erdogan of fomenting violence to gain votes for the ruling party.
No one has claimed responsibility, but the attack bears similarities to a suicide bombing the government blames on the Islamic State group, which killed 33 Turkish and Kurdish peace activists near a town bordering Syria in July.
Police detained 14 suspected Islamic State members Sunday in the central Turkish city of Konya, but it wasn't clear if they were related.
Turkey's government rejected the declaration, saying the rebels must lay down arms for good and leave Turkey. While no one group has been ruled out in the bombings, government opponents blamed security forces for failing to protect the peace rally.
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"The state which gets information about the bird that flies and every flap of its wing, was not able to prevent a massacre in the heart of Ankara," said Selahattin Demirtas, co-chairman of the pro-Kurdish People's Democracy Party.
Erdogan is hoping the ruling party regains its political majority, and critics accuse him of intensifying attacks on Kurds to rally nationalist votes. They worry the bombings could entice rogue Kurdish forces to attack, persuading Turks to seek security over peace.
The Islamic State group, which is fighting Syrian Kurdish forces allied to Turkey's Kurdish rebels, could benefit the most from this, since a continued military offensive within Turkey would take pressure off the extremist group in Syria.
Regardless of who may have planned the attack, it showed how deeply Turkey is being drawn into the chaos in Syria, with which it shares a 900 km-long border.