Authorities suspect the Islamic State (IS) group was behind an apparent suicide bombing in south-eastern Turkey that killed 31 people and wounded nearly 100.
The attack could represent a major expansion by the extremists at a time when the Turkish government is stepping up efforts against them.
Officials vowed to strike back at those behind the attack in the city of Suruc targeting a group of political activists who wanted to help the shattered Syrian city of Kobani, a bombing that turned a moment of hope into a scene of horror.
“We are face to face with a terrorism incident,” prime minister Ahmet Davutoglu said. “We have the willpower to find and certainly punish those who are responsible.”
There was no immediate claim of responsibility, but a senior government official said Turkey suspected IS was behind the blast as retaliation for Turkey’s steps against the militants.
The explosion took place as the Federation of Socialist Youths was wrapping up a news conference on plans to help rebuild Kobani, a witness said.
Suruc is just across the border from Kobani, the town that was the site of fierce battles between Kurdish groups and IS fighters.
The fall of Kobani, heavily populated by Syrian Kurds, was the biggest defeat last year for the militants since they established control over large parts of Iraq and Syria. Its ruins have become a symbol of Kurdish resistance.
If IS was behind the bombing, it would represent the group’s most serious attack inside Turkey. A female suicide bomber with suspected ties to IS blew herself up in a tourist district of Istanbul in January, killing a police officer and wounding another.
In recent weeks, Turkey has taken new steps against IS, blocking websites and arresting suspected followers in the country, officials said.
Witnesses to the latest blast described scenes of carnage and shock. Because the activists’ news conference was being recorded, the attack and its immediate aftermath were captured in widely circulated video.
Fatma Edemen said the federation of about 200 youths had been pressing for more access to Kobani to help with reconstruction.
The group was chanting “Long live the resistance of Kobani!” when the explosion tore through the crowd, she said.
IS militants carried out surprise attacks in Kobani last month that killed more than 200 people.
Turkish president Recep Tayyip Erdogan, who was in Cyprus on an official visit, was briefed on the investigation, according to the state-run Anadolu Agency.
“I personally and on behalf of my nation condemn and curse those who perpetrated this savagery,” Mr Erdogan said in a news conference broadcast on Turkish television.