They have poured into the streets enjoying basic rights they had been denied for two years, including shaving off their beards and smoking.
US-backed Kurdish and Arab fighters fought 73 days to drive IS out of Manbij, close to the Turkish border.
About 2,000 civilians being used as human shields were also freed.
Reuters news agency spoke to a resident of Manbij who described a spot where people were beheaded. "For anything or using the excuse that he did not believe [in God], they put him and cut his head off.
"It is all injustice," he said.
