Video footage taken in Moscow on Monday purports to show a woman in a hijab as she wanders near a train station carrying the bloody severed head of a child.
According to reports from the Russian capital, the woman was nanny to the 3- to 4-year-old girl before she beheaded her and then set her family's apartment on fire.
In thickly accented Russian, the woman--who is believed to be originally from Uzbekistan--screamed, "I am a terrorist, I want your death" Reuters reports.
The statement was part of a tirade in which the woman reportedly criticized democracy while she talked about the end of the world.
"According to preliminary information, the child's nanny, a citizen of one of the Central Asian states born in 1977, waited for the parents and elder child to leave the flat and then, for reasons not established, murdered the infant, set fire to the flat and left the scene," authorities in Moscow said in a statement translated from Russian.
The woman was reportedly living and working in Russia illegally.
While outside a Moscow metro station, passersby said the woman shouted "Allahu Akbar" as she paced with the child's head in her hand, the BBC reports.
Footage then appears to show a police officer wrestle the woman to the ground before she was taken into custody.
"Given the clearly deranged behavior of the detainee, investigators swiftly ordered her to undergo psychiatric tests to establish whether she is capable of understanding the significance of her actions," the statement reads.
The shocking scene came as Russians remain on high alert following October's attack that blew a Russian commercial flight out of the skies over Egypt's Sinai desert, killing all 224 onboard.