A woman was reportedly seen pulling the head out of a plastic bag after being stopped by police at the metro station.
She is reportedly shouted that she had killed the child and was seen praying shortly before police swooped.
The woman later told police she killedthe child because of her own husband’s infidelity, according to reports.
** WARNING: GRAPHIC CONTENT BELOW **
She is understood to have been wandering the streets for an hour before police arrested her.
At one point in the shocking video the woman is seeing raising the blood-covered head in the air while holding her index finger aloft.
The victim was identified as a young girl called Nastya M and her 39-year-old nanny has been arrested, according to reports.
The nanny has been identified as Gyulchehra Bobokulova, from Uzbekistan.
The station was closed after the woman shouted “I’m a terrorist” and threatened to blow herself up.
It was searched for explosives but given the all-clear by specialist officers a short time later.
Staff working in shops nearby the station were also ordered to evacuate.
Eyewitnesses at Oktyabrskoye Pole underground station said that the woman screamed:”My child was killed… I will blow up everyone.”
She is heard shouting that she has been “cursed” and “destroyed” “so many times,” before saying: “I hate democracy.
“I’m your suicide bomber… I’m going to die in a second. The end of the world.”
Witness Alyona Kuratova told Russia’s Rain TV: “I came out of the shopping centre and they tried to not let me through. There were a bunch of people there, running, screaming ‘bomb, bomb’.
“I saw that a woman was walking near the Oktyabrskoye Polye metro, near the road, all in black. She had not even some kind of bundle, but a head.
“Maybe it was a child’s head, like they were whispering in the crowd. She was shouting something.”
The woman has been detained by police, according to local reports.
The motive for the crime was unknown, police say.
Dramatic CCTV footage has captured the moment police swooped and dragged the woman to the ground in a tense stand-off.
A preliminary investigation revealed that the suspect is a children’s nanny in her late 30s, who is a citizen of a “Central Asian country”.
Investigators claim that the baby sitter waited until the parents of an older child had left the apartment before murdering the child and starting a fire.
A spokesman from the Investigative Committee of Russia said: “Given the clearly inappropriate behaviour of the person detained, investigators immediately appointed her to a forensic psychiatric examination to determine their ability to realise the significance of their actions and deeds.”
Fire crews rescued four people and put out the blaze, but then found the child’s beheaded body.
Yulia Ivanova, an official spokesperson for Moscow’s Interior Ministry, said: “[The suspect] had waited until the parents left with the older child, and for unknown reasons committed murder of the child, set the apartment on fire and left the scene of the crime.”
Nastya M, suffered learning difficulties
The dead child is said to have had learning difficulties and could not walk properly. Her parents had taken her to China for treatment, and were saving money to travel for subsequent care in Germany.