It happened around 10 p.m. Wednesday in the 2900 block of West Belden.
Witnesses saw the offender as he followed the woman to her home, then grabbed her and began sexually assaulting her in the gangway.
"She went into her gate and he forced himself in. When I saw that, I ran down the black and called the cops and described everything to them and started yelling outside trying to get a response from someone," said one witness.
Other neighbors also heard the woman's screams. They let their dog loose next door while they tried to chase the man away. Once the suspect noticed them coming toward him, residents say he ran away with his pants still at his ankles. He jumped a fence and started to run down the alley.
"I chased down the alley. He turned around. I was pretty close to him. A few feet away. He turned around and pointed the gun at me," said a witness.
At that point the neighbor says he backed away.
With the attacker still on the loose, the witnesses want to keep their identities from him.
In the meantime, police are working the neighborhood for clues.
A community policing meeting was scheduled for Thursday some time ago, but the nearby attack quickly became the prime topic of discussion.
" The fact that it happened at 10:00 at night with people still out is what is so amazing. The good thing is that there was an actual response from citizens of the community," said Sharif Bennett, Chicago Alternative Policing Strategy (CAPS) Volunteer
Police say they believe offender is between 17 and 20 years old. Witnesses saw a man matching that description following another woman several blocks south of the assault. Police are now hoping more witnesses will come forward to help them find the suspect.