Moscow police on Monday arrested a nanny for beheading a young girl in her care, with witnesses saying the black-clad woman was carrying a severed head and threatening to "blow everyone up".

The agitated woman, who was reportedly shouting "Allahu Akbar", was spotted pacing up and down outside a Moscow metro station in front of bewildered bystanders.

Russian investigators said a woman had been detained after killing a child aged three or four who reportedly had learning disabilities, then setting fire to the apartment and fleeing.

"I hate democracy. I am a terrorist. I want you dead," the woman in a headscarf shouted as she waved the head, according to footage broadcast by local television.

"You have become so hardened, you have eliminated so many of us. Look I am a suicide bomber, I will die, doomsday will come in a second," she said in accented Russian.

Witness Alyona Kuratova told independent Dozhd TV that the woman was holding the head by its hair.

DECAPITATED BODY

Kuratova described scenes of chaos, with police cars and ambulances arriving at the scene and some people yelling: "terror attack, terror attack."

She said she could not make out what the woman shouted but some media reported that she yelled "Allahu Akbar" — Arabic for "God is greatest" — and threatened to blow herself up.

Another witness said the woman shouted that she would "kill everyone, blow up everyone."

Some said the woman had paced up and down for some 20 minutes before she was detained near Oktyabrskoe Pole metro station. 

LifeNews, a news service with close ties to law enforcement agencies, said that when police approached her for a document check, she pulled the head out of her bag and started yelling that she had killed a child.

Television footage showed several men approaching and overpowering the woman, who some media reported suffered mental health problems.

"Monstrous and inexplicable," tweeted Russia's children's rights commissioner Pavel Astakhov, urging parents to carefully check on the mental state of nannies when they are hired.

Investigators said a nanny had been detained after the body of a child aged three or four years with "signs of violent death" was found on Monday in an apartment in northwest Moscow.

"According to preliminary information, the child's nanny... waited until the parents left the apartment with their elder child and, guided by unknown motives, killed the little one, set the apartment on fire and left the scene," the Investigative Committee said.

State news agency TASS, citing a law enforcement source, said: "The child's body was without the head. According to a preliminary version, the nanny carried away the head." 

Investigators said a criminal probe had been opened and that the suspect would undergo a psychiatric examination to establish whether she "understands the meaning of her actions and behaviour".

GRUESOME MURDER

Interfax news agency, citing a law enforcement source, identified the woman as 38-year-old Gyulchekhra Bobokulova, a native of Muslim-majority Uzbekistan.

LifeNews said Bobokulova had told authorities she committed the crime because of her husband's infidelity, adding that she may be suffering from manic psychosis.

The mother of the slain girl was taken by ambulance to an undisclosed location, Zvezda television channel said.

Media reports said the nanny had looked after the girl, who suffered a learning disability because of a medical mistake when she was born, for about 18 months.

Her parents had been collecting money to treat their daughter in Germany, mass-circulation Moskovsky Komsomolets reported.

Russians have in recent years seen all kinds of gruesome murders and attacks including the bombing of planes and the metro.

But the little girl's murder sent shockwaves across Moscow, with many saying the woman had not been apprehended quickly enough and questioning the professionalism of police.