At least 28 people were killed and over 60 wounded in the administrative heart of Turkey's capital Ankara on Wednesday when a vehicle laden with explosives detonated as military buses passed near the armed forces' headquarters, parliament and government buildings.The Turkish military condemned what it described as a terrorist attack on the buses as they waited at traffic lights.Justice Minister Bekir Bozdag said on Twitter the attack was an act of terrorism. Prime Minister Ahmet Davutoglu, who had been due to leave for a trip to Brussels later on Wednesday, canceled the trip, an official in his office said.There was no immediate claim of responsibility."I heard a huge explosion. There was smoke and a really strong smell even though we were blocks away," a Reuters witness said. "We could immediately hear ambulance and police car sirens rushing to the scene."Turkish media reports said many people were injured. 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.Images on social media showed the charred wreckage of at least two buses and a car. The explosion sent a large plume of smoke above central Ankara.Later on Wednesday, Swedish media reported an explosion severely damaged a Turkish cultural association building in a Stockholm suburb.All the windows of the center were blown out, newspaper Aftonbladet reported on its website, quoting police, adding there was no information about whether there were injuries."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.Turkey, a NATO member, faces multiple security threats. It is part of a US-led coalition fighting Islamic State in neighboring Syria and Iraq, and has been shelling Kurdish militia fighters in northern Syria in recent days.It has also been battling militants in its own southeast from the Kurdistan Workers Party (PKK), who have fought a three-decade insurgency for Kurdish autonomy. The PKK has frequently attacked military targets in the past, although it has largely focused on the mainly Kurdish southeast.Wednesday'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 labor 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 10 German tourists.