The woman was walking with her son and daughter when the four-year-old girl was snatched in Beeston yesterday.
The man approached them from behind as they were crossing Lodge Lane to Dawson Road at 11.30am.
He picked up the child and carried her across the road.
The mother went after him and challenged him and a passing van driver also intervened. The man released the child unharmed and ran off.
He was described as white, aged 25 to 40, about 5ft 9ins tall, large build, with short mousy brown hair. He was wearing black work trousers with pockets and a blue or black jacket. He appeared to have mental health problems.
Detective Inspector David Roberts, of Leeds District CID, said:
“This was obviously a very distressing incident for the young girl and her mother and we are treating it very seriously.
“We are carrying out extensive enquiries to identify the man involved and would like to hear from anyone who witnessed the incident or saw someone fitting his description in the area or who has any information that could assist the investigation.
“It is unclear at this time what the man’s intentions were and we are aware that he has been described as having mental health problems but the incident is clearly a cause for concern.
“We recognise that people in the area will be concerned and we are liaising with our colleagues in the local neighbourhood policing team who are increasing their patrols of the area to reassure the community.”
Anyone with any information is asked to contact Leeds District CID via 101 quoting log number 722 of April 26 or via www.westyorkshire.police.uk/101LiveChat or call the independent charity Crimestoppers anonymously on 0800 555 111.