Turkey's prime minister has implied the bombings that killed 17 people in Istanbul on Sunday were the work of the Kurdish separatist PKK group.
Recep Tayyip Erdogan said the attacks were a "cost" of the military campaign against the PKK based in northern Iraq.
The PKK has denied having anything to do with the bombings.
Mr Erdogan visited the bomb scene as a court considered whether to ban him and his AK Party from politics, for alleged anti-secular activities.
The Constitutional Court began its deliberations, on whether the AKP was trying to establish Sharia law by stealth, on Monday.
The party denies the charges. But the BBC's Sarah Rainsford, in Istanbul, says the chances are that the court will close down the AK Party, casting doubt over Turkey's political future, its international relations and hopes of joining the European Union.
It is not known whether Sunday's bombings were timed to coincide with the court case.
Nor has it been confirmed who was behind the attacks. However, the weight of blame on Monday appeared to be resting on the PKK.
Police said the attacks had the hallmarks of the group, and Istanbul Governor Muammer Guler said "there appears to be a link", though it remained unconfirmed.
Prime Minister Erdogan referred to the continuing air raid operation against PKK bases in Iraq, saying: "Unfortunately, the costs of this are heavy... The incident last night is one of them."
Sunday's first bomb was planted in a rubbish bin in the Gungoren area. It caused crowds to gather before a second, larger device detonated.
The blasts occurred about 10 minutes apart - the first at around 2200 local time (1900 GMT) on a busy pedestrian street.
There were scenes of panic, with people covered in blood as they tried to run from the scene. TV footage showed many victims lying on the street and being carried to ambulances in blankets.
Five children were among the dead. More than 150 people were injured. Some remained in critical condition on Monday.
As Mr Erdogan visited the scene, residents chanted: "Down with the PKK!"
The group has bombed Turkish civilians before. So have Islamist militants and radical leftists.
But the PKK leader Zubeyir Aydar was quoted by pro-Kurdish news agency Firat denying any involvement.
"The Kurdish freedom movement has nothing to do with this event, this cannot be linked to the PKK," he was quoted as saying.
"We think this attack was carried out by dark forces," he added, in what appeared to be a reference to a murky ultra-nationalist group that is under investigation for an alleged plot to provoke a military coup.
Meanwhile, the government itself was on trial in the Turkish capital Ankara.
It is the culmination of a series of clashes between the ruling AK Party, which has Islamist roots, and the secular elite.
The AKP won a resounding victory in elections just last year, but its critics say that does not mean it can bypass the country's strictly secular constitution.
More than 70 AKP members, including President Abdullah Gul and Mr Erdogan, could be banned from political activities for five years.
The AKP says the case is politically motivated.
But Mr Erdogan insisted the aftermath of the bombings, not the court case, was his priority.
"Our problem is not whether or not AK Party will be closed," he said.
"Our problem at the moment is to keep our unity so our country will go in a different direction."
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