NTV television said the bomb exploded close to bus stops near a park at Ankara's main square, Kizilay. The news channel said the explosion occurred as a car slammed into a bus, suggesting that the blast may have been caused by a car bomb. Several vehicles caught fire, it said.
Meanwhile, CNN is reporting that Twitter and Facebook have been suspeneded by the Turkish courts.
The explosion came just three weeks after a suicide car bombing in the capital targeted buses carrying military personnel, killing 28 people. A Kurdish militant group which is an offshoot of the outlawed Kurdish rebel group, the Kurdistan Workers' Party, or PKK, claimed responsibility for that attack.
Several ambulances and police cars were sent to the scene of Sunday's explosion. Police sealed off the area and pushed onlookers and journalists back, warning there could be a second bomb.
Ivory Coast Attacks
Meanwhile, 20 people - including 6 attackers - were killed in Grand-Bassam, a southeastern Ivory Coast beach town Sunday, in what is the third major attack on a tourism center in a West African country since November.
Three hotels were attacked in Grand-Bassam, which is a popular weekend destination for Ivorians and foreigners.
People ran from the beach amid the gunfire, as security forces responded and residents hid in their homes, a witness said.
An American embassy delegation was in Grand-Bassam on Sunday but the U.S. Embassy in Abidjan said it is monitoring the situation in Grand-Bassam and it has no evidence that U.S. citizens were targeted nor confirmed reports of any U.S citizens harmed.
Al Qaeda in the Islamic Maghreb, the North African branch of al-Qaida, claimed responsibility for the attack.
Meanwhile, French President Francois Hollande denounced the "cowardly attack."
"France will bring its logistical support and intelligence to Ivory Coast to find the attackers. It will pursue and intensify its cooperation with its partners in the fight against terrorism," Hollande said in a statement.
Attacks by extremists on hotels frequented by foreigners in two other West African countries, Mali in November and Burkina Faso in January, killed dozens of people and indicated that extremist attacks are spreading from North Africa.
The historic town of Grand-Bassam is a UNESCO World Heritage site.