A woman has been charged with murder in Russia after brandishing a severed child's head in public, stirring fears of an Islamist terrorist attack.

The woman, believed to be the victim's nanny and dressed in a hijab, was shown in footage published by online news portal Lifenews.ru on Monday holding the infant's head in her hands.

She is wrestled to the ground by a police officer. The portal said the woman shouted that she had killed the child.

With frequent warnings from government officials about the danger that Islamic State militants pose to Russia and a long history of terrorist attacks in Moscow, some onlookers thought they were witnessing an act of terror. 

One reporter, from the 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, the reporter, told Reuters.

"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 other footage of the scene, the woman can be heard shouting about the end of the world while proclaiming herself a terrorist. She reportedly also shouted "I want your death" and threatened to blow herself up.

She has been named by Russian media as Gyulchekhra Bobokulova, from the former Soviet republic of Uzbekistan, the BBC reports. Officials have not yet identified the suspect, however.

Investigators said the child was three or four years old and that the nanny had killed the child in the family's Moscow flat before setting fire to the premises and fleeing.

The motive for the crime was unknown, they said, and the woman was undergoing psychiatric testing to see if she was mentally sound. Russian news agencies cited an unnamed police source as saying the woman appeared to have been under the influence of psychotropic drugs. 

Additional reporting by Reuters.