Corpses of activists were seen strewn across the ground after the blasts, with the banners they had been holding lying next to them.
At least 30 people were killed and 126 wounded, the interior ministry said in a statement giving the first official toll.
"We curse and condemn this atrocious attack taking aim at our democracy and our country's peace," the statement said.
A Turkish government official told AFP that the authorities "suspect that there is a terrorist connection," without giving further details.
There were scenes of chaos after the blast, as ambulances searched for the wounded and police cordoned off the area.
"We heard one huge blast and then one smaller explosion and then there was a 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, in floods of tears.
Turkish police fired in the air to disperse demonstrators angered by the deaths of their fellow activists from the scene, an AFP correspondent said.
Amateur footage broadcast by NTV television showed smiling activists holding hands and dancing and then falling to the ground as the huge explosion went off behind them.
Initial reports spoke of a single explosion but Turkish media said later there had been two separate blasts in short sequence.
The authorities were exploring the possibility that the blasts could have been caused by a suicide bomber, the official Anatolia news agency said.
'Barbaric attack'
Prime Minister Ahmet Davutoglu had been briefed over the blast by Health Minister Mehmet Muezzinoglu, Anatolia said.
"We are investigating the explosion and will share our findings with the public as soon as possible," a Turkish official told AFP, without giving further details.
The area was to have hosted an anti-government peace rally organized by several leftist groups later in the day, including the pro-Kurdish Peoples' Democratic Party (HDP).
"We are faced with a huge massacre. A barbaric attack has been committed," said the HDP's leader Selahattin Demirtas.
The attack comes with Turkey on edge ahead of November 1 polls and a wave of unrest over the past few months.
An attack in the predominantly Kurdish town of Suruc on July 20 targeting pro-HDP activists and blamed on Islamic State (IS) jihadists killed 32 people and wounded a hundred others.
The militant Kurdistan Workers' Party (PKK) accused Ankara of collaborating with IS and resumed attacks on the Turkish security forces after observing a two-year ceasefire.
Over 140 members of the security forces have since been killed while Ankara claims to have killed over 1,700 Kurdish militants in weeks of bombardments of PKK targets in southeast Turkey and northern Iraq.
There had been suggestions that the PKK was about to announce a new ceasefire to help the HDP boost its score in the upcoming election.
The HDP performed strongly in the last vote on June 7, winning 80 seats to deprive President Recep Tayyip Erdogan's ruling Justice and Development Party (AKP) of an outright majority for the first time since it came to power in 2002.
The AKP then failed to form a coalition in months of talks, prompting Erdogan -- who had been hoping for a large majority to push through reforms to boost his powers -- to call another election on November 1.
The office of Davutoglu said that he had cancelled election campaigning for the next three days.
He was to host a meeting of top officials, including powerful spy chief Hakan Fidan, in the early afternoon to discuss the attack.
source: interaksyon.com
