The mother of the slain child said her daughter had been riding her bike about a meter away from her when it became stuck.
She saw the man approaching and at first thought he was going to help her daughter but he drew his knife and decapitated her.
"At first I thought he was going to lift up the bike to help my daughter. When I walked over to tell him that it's OK, I saw both of them fall on the ground, and the suspect was wielding the knife and cutting my daughter," she said.
"I saw the suspect slashing my daughter with a cleaver. I immediately grabbed him but I could not pull him away."
Bystanders heard her screams for help and rushed to restrain the attacker until police arrived.
"I'm devastated, because I will never see her again, and she'll never see her siblings again," the mother said.
She told reporters she hopes her daughter's death will prompt important discussions and "make people pay more attention to love."
An initial investigation found that the suspect, identified only by his surname Wang, waited near the metro station after allegedly buying the knife at a supermarket earlier that day.
The suspect was detained at a police station, where a large angry crowd gathered. Officers attempted to escort him out of the building, still with blood splashed across his face, but several people rushed the suspect, eventually forcing police to take him back inside.
