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A suspect (C) of decapitating a girl is escorted to Detention Center in Taipei, Taiwan March 29, 2016. REUTERS/Tony Huang

A four-year-old girl decapitated in an apparently random killing outside a subway station in Taipei has sparked an outpouring of grief and shock in Taiwan.

According to Taiwanese media, police have arrested the man believed to be responsible for Tuesday's shocking attack in Taipei. The suspect, a man surnamed Wang, had beheaded the child with a cleaver while she was with her mother.

Multiple reports note that the little girl, whose nickname was “Little Lantern” or “Little Lightbulb,” was riding her bike with her mother walking only a few feet behind her. They were on their way to the train station to pick up the little girl’s siblings and grandfather, SCMP reported. When the little girl’s bike got stuck on the street, the mother saw the man approach her daughter but thought he was trying to help her. Instead, he took a cleaver out and repeatedly struck the little girl.

The mother tried to stop the man but it was too late. Alerted by the mother’s cries, passersby helped to subdue the attacker until police arrived.

Police said Wang, who was beaten by onlookers as he arrived at a police station, had been arrested in the past over drug-related crimes. He had also been hospitalised in the past due to mental health issues.

"This is not a crime problem, but a societal problem," deputy director general of the National Police Agency, Huang Tsung-jen, told the BBC.

"The whole society's safety net has loopholes - we should be asking whether we have enough resources to help the mentally ill and the unemployed."

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A man leaves a message on a memorial book at a makeshift memorial near the site where a girl was found decapitated, outside a metro station, in Taipei, Taiwan March 28, 2016. Reuters/Tyrone Siu

A thorough investigation into the murder is now being conducted, by the order of Taiwan's outgoing President Ma Ying-Jeou.

Legislators have also proposed a bill that would sentence random killers of children to death following the shock attack.