By Julian Robinson and Will Stewart
A burka-clad babysitter decapitated the little girl in her care before walking through Moscow carrying the child's severed head, police say.
The woman shouted 'Allahu Akbar' as she appeared near Oktyabrskoye Pole metro station in the northwest of the Russian capital and threatened to blow herself up.
It came hours after officers found the headless body of a four-year-old child when they were called to a fire at a block of flats in the city.
A burka-clad babysitter has been arrested in Moscow after she walked through the streets carrying the severed head of a four-year-old girl
The victim was a girl identified as Nastya M - and the child's 38-year-old nanny Gyulchehra Bobokulova, from Uzbekistan, has been arrested.
The woman was seen pulling the severed head out of a bag and walking around near the entrance to the metro station as police moved in.
She is then said to have shouted that she had killed the child and was seen praying shortly before officers swooped.
According to local reports, she later told police she killed the girl because of her own husband's infidelity. Investigators immediately ordered a psychiatric test of the woman in a bid to understand her motives.
One eyewitness at the underground station told MK how the woman screamed: 'My child was killed … I will blow up everyone.' She also shouted: 'I hate democracy'.
A journalist working for RBC daily said she had heard the woman screaming 'Allahu Akbar' (God is Great).
'I was on my way to the metro station from home,' Polina Nikolskaya said.
'She was standing near the metro entrance and caught my attention because she was screaming Allahu Akbar.
'I saw that she had a bloodied head in her arms, but I thought it was not real. People in the crowd said it was real.'
In further footage from the scene, the woman can be heard shouting about the end of the world while proclaiming herself a terrorist.
The station was closed to passengers for some time, but no explosives were found on her.
Dramatic footage shows the moment police sprinted in towards the woman and tackled her to the ground.
Emergency services had earlier been called to an apartment nearby amid reports of black smoke billowing out of windows.
Firefighters rescued four people and put out the blaze - but then found the child's beheaded body.
Investigators claim that the babysitter waited until she was along with the child in the apartment before carrying out the murder and starting a fire.
The source in the Investigative committee told TASS: 'She waited until the parents with the elder child left the flat, then for unknown reason she killed the child, set fire to the apartment and left the scene.
'She was detained at the metro station Oktyabrskoe Pole.'
The dead girl, who is said to have had learning difficulties and could not walk, had a 15-year-old brother. Her family, from the Oryol region, was renting the apartment.
She suffered damage because of birth problems in August 2011.
The family took her for treatment to China - and were saving money to travel for subsequent care in Germany.
The girl's mother, who works in a wedding shop, was rushed to hospital in an unconscious condition after learning of her daughter's death. The child's father is a technician at a mobile phone company.
The parents told police the nanny had been working for them for 18 month.
Sources say the babysitter told interrogators she did not want to hide from police, and aimed to draw maximum attention to what she has done.
She had not intended to ignite the flat deliberately and destroy evidence, she said, according to the source, and wanted the parents to know who had killed their daughter.
The woman had a valid residency permit for Russia but was working illegally. She had no work permit, said officials.
The nanny faces up to 20 years in jail if she is deemed psychologically fit to stand trial.
This afternoon, the investigation was taken over by the FSB, Russia's powerful domestic secret service, once headed by Vladimir Putin. Police are not currently treating the incident as terrorism.
LifeNews claimed: 'Investigators suspect that Gulchekhra had manic psychosis developing for a period of time.'
A law enforcement source said the woman was suspected of handling 'explosive elements' at the flat which was engulfed in flames.
But 'it might be an inflammable liquid that she used to set the apartment on fire where she killed four-year-old Nastya M.'
The security services are hunting for the woman's husband. A source said she may have been on 'light drugs'.