A senior Turkish official has acknowledged that 35 civilians were killed in an air strike near a Kurdish village close to the border with Iraq.
Turkey's military said earlier it had targeted suspected Kurdish militants.
But the victims of Wednesday night's attack are believed to have been villagers involved in smuggling cigarettes into Turkey from Iraq.
Governing party vice-president Huseyin Celik said an investigation was looking into possible intelligence failures.
The attack, on Wednesday night, took place near the village of Uludere in Sirnak province in south-eastern Turkey.
In a statement, Turkey's general staff said the area attacked on Wednesday night was inside northern Iraq and had no civilian population. It added that the raid was launched following information that suspected militants were planning to attack Turkish security bases.
But Mr Celik was quoted by AFP news agency as saying later that "if it turns out to have been a mistake, a blunder, rest assured that this will not be covered up".
The pro-Kurdish Peace and Democracy Party condemned the air strike as a "massacre", saying that all the victims were civilians aged between 16 and 20.
"Those killed were young people who made a living from smuggling. There were people studying for university exams among them," said party leader Selahattin Demirtas.
Mr Celik added that some of the victims were sons of village guards who had helped Turkish troops in their fight against the rebels.
The opposition Republican People's Party (CHP) urged the military to exercise greater care. While smuggling was against the law, "being killed in an aerial bombardment was not what they deserved", a leading CHP official told the Hurriyet website.
Protesters angry at the deaths threw stones at police close to Taksim Square in Istanbul early on Thursday evening, Turkish agencies report.
Provincial governor Vahdettin Ozkan said initially that more than 20 people had lost their lives but his office later clarified that 35 had been killed and one injured.
"A crisis centre is being formed at the scene and prosecutors and security officers were sent there," he told Anatolia news agency.
The mayor of Uludere was quoted by Reuters news agency as saying that all the victims had suffered from burns.
Local officials said drums of diesel carried by the group had exploded.
Many of the victims were said to have been related.
Those killed had been using mules to cross the border when the incident happened, they said.
"We were on our way back when the jets began to bomb us," a survivor, Servet Encu, told the pro-Kurdish Firat news agency.
The group had been waiting on the Iraqi side of the border for some time about 3km (2 miles) from their village, because they had been told that the route was blocked by the military, Turkish media report.
Pictures from the scene showed bodies on a hillside covered in blankets. The bodies were later taken away to a local hospital.
Smuggling of fuel and cigarettes is said to be commonplace between villages along the Iraqi border. But rebels from the Kurdistan Workers Party (PKK) have crossed the border into Turkey to stage attacks on Turkish forces.
After 24 Turkish soldiers were killed in PKK raids in October, Turkish forces responded with a series of air and ground attacks.
- 22 October 2011
- 19 October 2011