The child's aunt had left two-year-old Yana Leonenko unattended for several minutes in a pushchair when the 27-year-old woman, named only as G. Sharaliyeva, seized her.
The aunt returned to the pushchair to see a woman dressed in a white T-shirt and black trousers in the distance with her niece. I pleaded for help, and ran after the woman, but fell down,' she said.
No-one helped her chase the woman, she said, while eyewitnesses said the girl had easily gone with the kidnapper.
The suspect dragged the girl through a park, and later carried her, say investigators, highlighting CCTV footage.
A Muslim, she did not normally go outside without covering her hair, but chose to ditch her hijab, it is alleged. Then she took a taxi and vanished. Later the girl's body was found drowned in a river running through a park in the the city of Ivanovo.
A court ordered Sharaliyeva to be detained with investigators alleging she killed her husband's love child. The suspect was held in a metal cage in the court room.
A spokesman said: 'The investigation believes that the person who kidnapped and murdered the child is the wife of the biological father of the girl. The woman made use of the short absence of a relative of the child and kidnapped the girl from the entrance to a residential building.'
A family friend said the woman's husband Sakhyd Sharaliyev had had a relationship with a waitress in his cafe, resulting in the pregnancy.
They explained: 'They had an affair three years ago. She was working as a waitress in his cafe. Soon she got pregnant.
'The man didn't want to leave his wife and their two children but promised to help to raise the child.' The wife had been oblivious her husband had cheated on her.
'His wife came from Azerbaijan about four years ago and for a long time didn't know her husband had a mistress, more than one, in fact.
'He is very kind, and helpful but scatterbrained. He was cheating his wife but she simply didn't know about it.'
The husband admitted he had fathered the child, and denied keeping Yana's existence a secret from his wife, with whom he has children aged four and six.
'When they said it's my wife, I was shocked,' he said.
She never went out without covering her head and she seemed 'naked' to him in the CCTV images, he said.
Her hair isn't covered...she has no such clothes at home.'
He insisted: 'She was always quiet and calm, she has never said anything (angry about the baby).
'I promised her to not see that woman (Yana's mother) again, but to help the child.'
An eyewitness who saw the child as she was abducted said: 'The girl wasn't crying, wasn't fighting.'