PHOTO: EUROPEAN PRESSPHOTO AGENCY
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.
Read: 28 dead in suicide attack in Turkey border town
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.
AFP pictures showed bodies covered in blankets lain out in the centre's garden.
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 spokesperson 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.
White House spokesperson Josh Earnest condemned the "heinous" attack, as did Russian President Vladimir Putin, who labelled it a "barbaric act" and called for greater international cooperation in fighting terrorism.
French Foreign Minister Laurent Fabius also joined in the condemnation.
The attack in Suruc was followed closely afterwards by a suicide car bombing at a checkpoint in Kobane, which killed two members of the Kurdish security forces, according to Rami Abdel Rahman, director of the Syrian Observatory for Human Rights.
Kobane has been a symbol of resistance against militants since IS fighters were driven out by Syrian Kurdish forces backed by US-led airstrikes.
Read: Suicide attack on Turkish embassy vehicle kills two in Kabul
Turkey's Kurds were frustrated at the time at Ankara's refusal to intervene to rout the insurgents, who have seized large parts of Syria and Iraq over the past year.
Ankara's critics accused it of tolerating or even aiding IS, as a useful ally against Syrian President Bashar al-Assad, whom Erdogan wants ousted -- allegations vehemently rejected by Ankara.
In recent weeks, Turkish authorities have stepped up their actions against the jihadists, arresting dozens of suspected IS militants and sympathisers.
"It's now obvious that the Turkish government has upgraded the threat posed by ISIS to among the top ones it is facing," a Western diplomat told AFP last week.
Turkey has also boosted its border defences, stationing tanks and anti-aircraft missiles along its frontier with Syria as well as bolstering troop numbers.
The build-up has fed speculation that the government is planning an intervention to push the jihadists back from the border and halt the advance of Kurdish forces who have made gains in the area.
The government has however ruled out any immediate action in Syria.
Ankara categorises IS as a terrorist group but has been a reluctant member of the US-led anti-IS coalition, refusing to give its NATO ally the use of Incirlik air base in the south for raids on the militants.
The militants made a surprise raid on Kobane last month, five months after being driven out of the town.
The nearby town of Suruc, once a centre of silk-making, is home to one of the biggest refugee camps in Turkey housing Syrians who have fled their country's bloody four-year conflict.
The camp shelters about 35,000 refugees out of a total of more than 1.8 million refugees taken in by Turkey since 2011.
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