Children were among at least 56 civilians killed Tuesday in raids by coalition warplanes as they fled the village of Al-Tukhar near the key IS stronghold of Manbij, according to the Syrian Observatory for Human Rights.
Facebook pages managed by Syrian activists urged people around the world to take to the streets to protest the deaths under the banner "Manbij is being exterminated".
"We ask all Syrians, whatever their affiliations or sects, and all free people of the world and especially the people of Manbij to stand in solidarity with our devastated city on Sunday, July 24," wrote one page that publishes local news about Manbij.
It said it was calling for demonstrations in reaction to "the massacres carried out by coalition warplanes, with the latest... in Al-Tukhar".
The village lies 14 kilometres (nine miles) north of Manbij, which has been repeatedly targeted by coalition raids in support of an offensive to wrest the town from IS control.
Activists on the "Manbij News" page called for protests in several Turkish cities including Istanbul and Gaziantep.
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Another Manbij-affiliated page posted photographs of demonstrators gathering on Wednesday in the rebel-held town of Azaz, further west, to condemn the raids.
"Our children will tell God everything," read one Arabic-language sign held by a young boy.
"The Al-Tukhar massacre is a stain of shame on humanity," another sign said.
Asked about the Al-Tukhar raids, the US-led coalition said it had recently "conducted air strikes near Manbij" and was looking into the reports of civilian casualties.
The raids also drew condemnation from the UN's children agency, which said that "more than 20 children were reportedly killed" in the raids on Al-Tukhar.
"Such horrific incidents confront parties to this conflict with their shared responsibility to respect international humanitarian laws that protect children in war," said UNICEF's Syria representative, Hanaa Singer.
"No matter where they are in Syria or under whose control they live -- absolutely nothing justifies attacks on children."