The blast ripped through the centre in Suruc -- a town just across the border from Kobane, which was itself later hit by a suicide car bombing -- blowing out the windows and starting a fire, witnesses said.
Most of the dead were university students who were planning to enter Syria to help rebuild Kobane, which was occupied by Islamic State for months before being recaptured by Kurdish forces in January.
President Recep Tayyip Erdogan, on a visit to northern Cyprus, condemned the attack as an "act of terror".
"On behalf of my people, I curse and condemn the perpetrators of this brutality," he said. "Terror must be condemned no matter where it comes from."
Television footage showed several people lying on the ground covered in blood and ambulances rushing to the scene.
Prime Minister Ahmet Davutoglu pointed the finger of blame for what was "clearly a terrorist attack" at Islamic State.
"Preliminary findings point to it being a suicide attack carried out by Daesh," Davutoglu said in Ankara, using an Arabic acronym for IS. "But we are not at a point to make a final judgement."
If confirmed, it would be the first such attack by IS fighters against Turkey, a regional military power and NATO member.
Local resident Mehmet Celik told AFP the town was "in chaos".
Alp Altinors from the pro-Kurdish HDP party said the group of around 300 activists who gathered in Suruc from across the country were from the Federation of Socialist Youth Associations and that most were students.
"They were planning to build parks in Kobane, hand out toys for children and paint school walls," he told AFP.
Social media images showed the group relaxing over breakfast in the garden a few hours before the noon blast.
A video circulated by the private Dogan news agency showed a spokesman for the activists saying into a microphone: "We, the youth, are here. We have defended Kobane together and now we are setting out to rebuild it together."
Davutoglu said the blast aimed to undermine Turkish democracy.
"This attack targets us all," he said, dispatching three ministers to the southeastern region.
"Daesh threatens not only Syrian people but also Turkey," he added.
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