HONG KONG • A 37-year-old woman has been arrested on suspicion of killing and dismembering her 12-year-old daughter, in a gruesome crime that has shocked Hong Kong.

The killing was discovered on Sunday night when police visited the tiny flat mother and daughter lived in over an unrelated complaint, Hong Kong media reported.

They found the girl's remains in the bathroom, Mong Kok assistant district commander Chung Chi Ming said.

"Her head, organs and body had been mutilated. The bathroom had flooded, which was affecting neighbouring flats," Mr Chung told reporters late on Sunday.

He said a meat cleaver, believed to have been used on the body, was found in the flat.

"No illegal drug was found, but an instrument used to take the drug Ice was discovered," he added.

The mother, identified by police as a mainland Chinese woman surnamed Cao, was found delirious by police after they forced their way into her flat in the Man Fook Building, which is near the Mong Kok police station.

Cao was detained for questioning. Police later removed 11 bags of body parts from the 5.6 sq m flat, one of several subdivided units in the building.

The daughter was identified only by her surname He. She was born in China and had been going to school in Hong Kong.

Teachers at the nearby Tong Mi Road Government Primary School where the girl was studying described her as a pupil with "good conduct" who had a "good relationship with teachers and schoolmates".

When asked if Cao appeared delirious or had resisted when she was arrested, Mr Chung would say only that she was not able to cooperate with officers at the time.

"We have to wait until her situation gets better to investigate," he said.

Police said there was no record of Cao's mental history or reports of domestic violence.

Cao had come to Hong Kong with her daughter in 2013 after marrying her second husband, a Hong Konger, reported Apple Daily, citing sources.

However, she eventually divorced her second husband and worked as a masseuse to support herself and her daughter, the newspaper said.

Her first husband, who is the girl's father, lives in China, the report said. Police were trying to get in touch with him.

Cao's neighbour told reporters yesterday that she heard chopping noises from the flat at around 3pm on Sunday.

A strong antiseptic smell then wafted from the flat.

The neighbour, named only as Ms Tsui, said she often saw the suspect about but had never met the daughter.

Police were called to Cao's flat on Sunday night by a neighbour who had lent a mobile phone to Cao but was unable to get it back from her.

When police arrived at the scene at around 9pm, they had to break down the door as Cao refused to let them enter, media reports said.

The flat was flooded with water mixed with blood. Police then found dismembered body parts in the bathtub.

Drainage pipes were reportedly clogged because the suspect tried to get rid of the body parts by flushing them down the pipes.

Yesterday, forensic pathologists took away other items, including bloodstained jewellery and laundry detergent, reports said.

Police were also looking at closed-circuit television footage of the shops in the neighbourhood.