The attack, near Ankara's main train station, ratcheted up tensions ahead of Turkey's November 1 snap elections which were already soaring amid the government's offensive on Kurdish militants.
Bodies of the slain activists were seen strewn across the ground after the blasts, with the banners they had been holding for the "Work, Peace and Democracy" rally lying next to them.
Turkey's President Recep Tayyip Erdogan denounced the "heinous attack", saying it was aimed at "our unity and our country's peace."
Prime Minister Ahmet Davutoglu said there were "strong signs" that the attack had been carried out by two suicide bombers.
With the country shattered by the deadliest attack in the history of modern Turkey, Davutoglu declared three days of national mourning.
"We heard one huge blast and then one smaller explosion and then there was a great movement and panic. Then we saw corpses around the station," said Ahmet Onen, 52.
"A demonstration that was to promote peace has turned into a massacre, I don't understand this," he said, sobbing.
Turkish police fired in the air to disperse demonstrators angered by the deaths of their fellow activists from the scene, an AFP correspondent reported.
Davutoglu said no group had claimed responsibility for the bombings. But he said groups including Islamic State (IS) jihadists, the Kurdistan Workers Party (PKK) and the far-left Revolutionary People's Liberation Party-Front (DHKP-C) were capable of carrying out such an attack.
Reports said that hundreds of people in Ankara had rushed to hospital to donate blood for the victims.
