Ankara Gov. Mehmet Kiliclar said authorities believe that the explosion was caused by a "bomb-laden vehicle."
News reports said some cars caught fire and dozens of ambulances were sent to the scene. Dark smoke could be seen billowing from a distance.
There was no immediate claim of responsibility, but government officials said they were treating the incident as a terrorist attack. Kurdish rebels, the Islamic State in Iraq and the Levant (ISIL) and a leftist group have carried out attacks in the country recently.
In October, suicide bombings blamed on ISIL targeted a peace rally outside the main train station in Ankara, killing 102 people in Turkey’s deadliest attack in years.
Wednesday’s attack comes at a tense time, with the Turkish government facing an array of challenges. A fragile peace process with Kurdish rebels collapsed in the summer. The Turkish security forces have been engaged in large-scale operations against Kurdish fighters in the southeast since December, imposing controversial curfews in flashpoint areas, and the fighting has displaced tens of thousands of civilians.
Turkey has been helping efforts led by the United States to combat ISIL in neighboring Syria and has seen several deadly bombings in the last year that were blamed on the armed group.
The Syrian war, meanwhile, is raging along Turkey's southern border. Recent airstrikes by Russian and Syrian forces have prompted tens of thousands of refugees to flee to the Syria-Turkey border. Turkey so far has refused to let them in, despite being urged to do so by the United Nations and European nations, but is sending aid to refugee camps on the Syrian side.
Turkey, which is already home to 2.5 million Syrian refugees, has been a key focus of EU efforts to halt the biggest flow of refugees in Europe since World War II. Hundreds, sometimes thousands, of refugees leave every night from Turkey to cross the sea to Greece in smugglers' boats.
NATO Secretary General Jens Stoltenberg strongly condemned the "terrorist attack" and offered his condolences to the families of the victims. He said that there can be no justification "for such horrific acts" and that "NATO allies stand shoulder to shoulder in the fight against terrorism."
German Chancellor Angela Merkel said, "In the battle against those responsible for these inhuman acts, we are on the side of Turkey."
Washington also condemned the attack, according to a statement by Mark Toner, the State Department deputy spokesman. "We reaffirm our strong partnership with our NATO ally Turkey in combating the shared threat of terrorism," he said.
Turkish Prime Minister Ahmet Davutoglu canceled a visit to Brussels on Wednesday evening and attended a security meeting with President Recep Tayyip Erdogan and other officials.
The government imposed a gag order barring media organizations from broadcasting or printing graphic images of the dead or injured at the scene of the explosion. Turkey has imposed similar bans after previous attacks.
Last month 11 German tourists were killed after a suicide bomber affiliated with ISIL detonated a bomb in Istanbul's historic Sultanahmet district. More than 30 people were killed in a suicide attack in the town of Suruc, near the border with Syria, in July.
Al Jazeera and wire services