The explosions occurred minutes apart near Ankara's main train station as people were gathering for the rally, organized by the country's public sector workers' trade union and other civic society groups. The rally aimed to call for an end to the renewed violence between Kurdish rebels and Turkish security forces.
It was not clear if the attacks, which came weeks before Turkey's Nov. 1 elections, were suicide bombings.
An Associated Press photographer at the scene reported seeing several bodies that were covered with flags and banners that demonstrators had brought with them for the rally. Police later cordoned off the area.
Television footage from Turkey's Dogan news agency showed a line of protesters fanned out on the street near the train station, chanting and performing a traditional dance with their hands locked, when a large explosion hit behind them.
The video also showed several people lying injured on the streets or being taken into ambulances.
An Interior Ministry statement condemned the attack which it said "targets Turkey's democracy and peace." Prime Minister Ahmet Davutoglu called an emergency security meeting to discuss the attack. His office said he was suspending his election campaign programs for the next three days. President Recep Tayyip Erdogan cut short his program in Istanbul to return to the capital.
"There was a massacre in the middle of Ankara," said Lami Ozgen, head of the Confederation of Public Sector Trade Unions, or KESK. "Two bombs exploded in very short intervals."
There was no immediate responsibility claim. A government official said authorities were investigating the blast.
"The toll is severe," said Selahattin Demirtas, leader of Turkey's pro-Kurdish party, whose members attended the rally. "There are dozens of dead and hundreds of injuries. Some of our friends are in serious condition."
In July, a suicide bombing blamed on the Islamic State group killed 33 people in a town near Turkey's border with Syria.
A leftist militant group has also carried out suicide bombings in Turkey.