At least 17 people were killed on Sunday in a car bombing near a bustling market and mosque in the northern opposition-held Syrian city of Azaz, said the Syrian Observatory for Human Rights.
Four children were among those killed in the explosion in Azaz, at the heart of a Turkish zone of influence in Aleppo province, it reported.
"Many people were leaving evening prayers when the explosion happened," Observatory head Rami Abdul Rahman told AFP.
The attack also wounded more than 20 people, according to the Britain-based monitor.
In a nearby market, shoppers purchasing clothes and gifts ahead of the upcoming Eid al-Fitr holiday, which marks the end of the Muslim holy month of Ramadan, were also among the victims, said Jihad Berro, a coroner at a local hospital.
He said the medical facility was crammed with victims and their relatives.
"The emergency rooms were full, we placed corpses on the floor," he said.
"It is a real catastrophe before the Eid al-Fitr holiday."
Resident Abu Youssef said the explosion torched the neighborhood.
"The damage is severe: at least six stores were burned, the storefronts of dozens of other stores were blown out," he told AFP.
It was not clear who was behind the attack.
The bombing comes one day after a similar explosion killed 10 people and wounded 20 in the northeastern city of Raqqa.
Turkey launched Operation Euphrates Shield in 2016 and seized more than 2,000 square kilometers in northern Syria including Azaz, clearing the area of ISIS extremists, while preventing any Kurdish advance in the region.
Ankara keeps Turkish troops and intelligence forces in the area, and still backs the local police forces.