Several hundred protesters had gathered in Turkey's largest city to show support for at least 31 victims of a blast that ripped through a rally earlier on Monday in the Turkish border town of Suruc.
The Turkish government is calling the explosion a suspected suicide bombing.
At least 100 others were wounded, provincial Governor Izzettin Kucuk told Turkish media.
The explosion occurred at midday at the Amara Cultural Park in Suruc, where a group had gathered calling for more help to rebuild the Syrian Kurdish city of Kobani, CNN Turk reported.
Suruc is about six miles from the border and Kobani, the Syrian city that was the scene of intense fighting between Syrian rebels and Kurdish forces and ISIS
Photos and video taken from the scene show bodies strewn around a park and dazed people at the blast site while emergency teams rush to aid victims.
"Kids were screaming, everyone was screaming and running," eyewitness Erhan Subasi told CNN. "Everyone was in a panic but trying to help.
"People's faces were blown off, arms scattered ... you could smell burned bodies."
There was no immediate claim of responsibility, but Prime Minister Ahmet Davutoglu suggested ISIS was responsible for the attack.
He urged the country to unite and said the government would not hesitate in taking action in response to the bombing.
A Turkish official speaking to CNN on condition of anonymity said the government thinks the attack is "retaliation to the Turkish government's fight against terrorism."
The official called for the international anti-ISIS coalition to "more actively pursue a solution to the crisis in Syria
Turkish President Recep Tayyip Erdogan condemned the attack.
Terror doesn't have a nation, race or homeland," he said. "That is why we have been consistent in underlining the need for a global effort to fight terror."