She was on her way to meet a friend of the same age, who lives in the central city district St. Eriksplan, and it was just outside her friend's house that the young girl was suddenly attacked.
"A call was made to the police at 8:24pm," Stefan Nordenmark of the Stockholm police force's communications central told the newspaper Expressen.
The girl was badly shocked, reported the newspaper, but was able to tell police that an unknown man had attacked her and then molested her.
"A man, of no acquaintance to the girl, raped her in a stairwell," said Anders Molin, station officer of the Norrmalm police, to news agency TT.
The event has sparked a major police search. Some 30 officers were involved in the search for the perpetrator on Friday night.
As of Saturday morning, however, no suspect had yet been arrested.
"We're going to continue our work with interrogations and go through all the tip-offs and leads we've received," said Anna Olsson of the Norrmalm police to Expressen.
Rapes of this sort, against young children, are very unusual in Sweden, according to statistics from Sweden's National Council of Crime Prevention (Brottsförebyggande rådet - BRÅ).
2010 saw a total of 23 attempted rapes outdoors against girls under 15 years of age, a figure which has hovered around the same number for the past couple of years.