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 metro station in the northwestern part of the Russian capital, witness Alyona Kuratova told Dozhd TV, saying she was holding the head by its hair. 

"I saw that it was indeed a head," she told the independent channel. "Maybe it was a child's head as they started whispering in the street."

She described scenes of chaos as police cars and ambulances began arriving at the scene, with several people yelling: "terror attack, terror attack."

"The woman was going back and forth," Kuratova said.

"She yelled something."

The witness 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.

LifeNews, a news service known for its 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.

Footage broadcast by Russian television showed several men approaching and overpowering the woman. As she fell down, an object she carried around rolled along the ground.

Russian 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 after a fire was extinguished there.

"According to preliminary information, the child's nanny -- a native of one of the Central Asian countries, born in 1977 -- 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 in a statement.

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

 'Monstrous and inexplicable'

 Investigators said the woman was detained and a criminal probe opened. They said the suspect would undergo a psychiatric examination to establish whether she "understands the meaning of her actions and behaviour."

The Investigative Committee did not provide further details.

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

The mother of the slain girl was taken away by an ambulance in a state of unconsciousness, state Zvezda television channel said.

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

"Papers from the narcologist and psychiatrist should be obligatory when you hire a nanny," he said.