A suspected suicide attack has killed at least 27 people and wounded nearly 100 others in the Turkish town of Suruc near the Syrian border.
The blast occurred in the garden of a cultural centre at about 09:00 GMT, the interior ministry said.
Hundreds of young people were staying there to assist in rebuilding work in the nearby Syrian town of Kobane.
Turkish officials told Reuters initial evidence indicated it was a suicide attack by the Islamic State (IS) group.
Suruc has housed many refugees who have fled fighting in Kobane between IS militants and Kurdish fighters since September last year, when IS overran the town.
The Federation of Socialist Youth Associations (SGDF) is reported to have had at least 300 members staying at the Amara Culture Centre to take part in rebuilding work in Kobane.
A photo taken prior to the blast showed the group in the garden having lunch.
A statement from the interior ministry said: "We call on everyone to stand together and remain calm in the face of this terrorist attack which targets the unity of our country."
Pervin Buldan, a senior lawmaker from Turkey's pro-Kurdish HDP opposition party, said: "Turkish and Kurdish youth had come to cross into Kobane, and there were three or four days of activities planned."
ANALYSIS: JIYAR GOL, BBC REGIONAL EXPERT
The suicide bomb attack on the Amara Cultural Centre is one of the bloodiest suicide attacks in Turkey in years.
Suruc is a small Kurdish-majority city just a 15 minute drive from the border with Kobane. Kurdish activists in Suruc played a vital role during the siege of Kobane, sending food and medicine to the YPG Kurdish fighters to bolster their supplies.
Many journalists and foreign fighters who wanted to go to Kobane went to Suruc and from there were sent on.
At the time of the attack, 300 young activists were preparing to make a statement and cross the border into Kobane to help to rebuild the city.
Local Kurdish politicians in Suruc blame the Islamic State (IS) group for the attack. IS suffered a heavy loss and defeat in Kobane earlier this year.
Also last month the YPG captured Tal Abyad, one of the most important IS border crossings with Turkey. Kurds believe the militant group wants to take revenge on civilian Kurds inside Turkey.
In June it carried out numerous attacks on Turkey's pro-Kurdish Party, HDP, during the run-up to the Turkish parliamentary elections. IS is believed to have many sympathizers inside Turkey and they could carry out attacks against additional targets.