Twenty-eight people were killed and dozens wounded in Turkey's capital Ankara when a car laden with explosives detonated next to military buses near the armed forces' headquarters, parliament and other government buildings.
The Turkish military condemned what it described as a terrorist attack on the buses as they waited at traffic lights in the administrative heart of the city.
A government spokesman said 28 people had been killed and 61 wounded in the blast, which took place near a busy intersection less than 500 metres from parliament during the evening rush hour.
Justice Minister Bekir Bozdag described the attack as an act of terrorism and told parliament, which was in session when the blast occurred, that the car had exploded on a part of the street lined on both sides by military vehicles.
Prime Minister Ahmet Davutoglu, who had been due to leave for meetings in Brussels later, cancelled the trip, an official in his office said.
President Recep Tayyip Erdogan postponed a planned visit to Azerbaijan.
President Erdogan vowed that Turkey was more determined than ever to use its right to "self-defence" after the attack.
"Our determination to respond in kind to attacks taking place inside and outside our borders is getting stronger with such acts," Mr Erdogan said in a statement.
"It must be known that Turkey will not shy away from using its right to self-defence at any time, any place or any occasion."
A senior Turkish security source said initial signs indicated that Kurdish militants from the outlawed Kurdistan Workers Party (PKK) were responsible.
Separate security sources in the mainly Kurdish southeast, however, said they believed so-called Islamic State militants may have been behind the bombing.
There was no immediate claim of responsibility.
A health ministry official said the authorities were still trying to determine the number of dead and wounded, who had been taken to several hospitals in the area.
The explosion sent a large plume of smoke abovecentral Ankara.
Turkey, a NATO member, faces multiple security threats.
It is part of a US-led coalition fighting Islamic State in neighbouring Syria and Iraq, and has been shelling Kurdish militia fighters in northern Syria in recent days.
It has also been battling PKK militants in its own southeast where a two-and-a-half year ceasefire collapsed last July, plunging the region into its worst violence since the 1990s.
The PKK, which has fought a three-decade insurgency for Kurdish autonomy, has frequently attacked military targets in the past, although it has largely focused its campaign on the mainly Kurdish southeast.
Today’s bombing comes after an attack in Ankara in October blamed on Islamic State, when two suicide bombers struck a rally of pro-Kurdish and labour activists outside the capital's main train station, killing more than 100 people.
A suicide bombing in the historic heart of Istanbul in January, also blamed on Islamic State, killed ten German tourists.
Kurdish militants, radical leftists and Islamic radicals, have all staged bombings in Turkey in recent years.
Ankara attack forces cancellation of refugee summit
A mini-summit on Europe's refugee crisis gathering 11 EU countries and Turkey, due to take place tomorrow, has been cancelled because of the bomb attack in Ankara, diplomats said.
The talks, taking place ahead of a full summit of the 28-nation European Union, were to gather the leaders of 11 EU countries and Mr Davutoglu to focus on how to resettle Syrian refugees who have fled to Turkey.
Blast hits Turkish cultural centre in Sweden
An explosion has severely damaged part of a building that housed a Turkish cultural association in a Stockholm suburb tonight but no one was injured, police said.
Police said all the windows of the centre were blown out and that technicians were on site to investigate the cause.
The centre was located in the basement of a building in Fittja, in southwest Stockholm.
"No one was inside. No one was injured. It had been locked since earlier in the evening," a police spokesman said.
No one has been arrested and there are currently no suspects, he added.