Pueblo police later released the man because of lack of evidence, The Pueblo Chieftain reported.
Neighborhood residents were looking for a man suspected of two separate sexual acts when they got word that a man matching the description had been spotted, said Alex Pacheco, one of the pursuers.
The group confronted the man and he ran.
Pursuers surrounded him and punched him in the face, police Capt. Tom Rummel said. Arriving officers shoved the man into a police car and whisked him to the station for questioning. He was not seriously injured.
"The primary officer on the scene said get him out of here," Rummel said.
Pacheco told the newspaper that residents were canvassing the area looking for the man who committed the sex crimes during the past few months.
One incident involved the sexual assault of a girl in her home. In the other, authorities said a man with the same description exposed himself to another child.
Police said the mob grew to about a half-dozen people as residents learned of the chase and joined in.
"We went through the right channels in contacting the police but there hasn't been much response," Pacheco said. "We can't wait around any longer without doing something. These are children that this man is after and we can't let any more children get hurt by him."
Rummel said police had notified the media and posted warnings on social media about the attacks, but authorities are not required by law to notify residents because no one had been arrested.
Rummel said police only had a vague description of the suspect because he wore a bandanna over his face.
The 54-year-old man accosted by the mob did not want to file charges against his pursuers, the chief said.
"He said folks were reacting to a bad situation and he told the officer, 'I don't want to go that route,'" Rummel said. "He was just in the wrong place at the wrong time."
The name of the man was not released because no charges were filed. He agreed to give investigators a DNA sample so he could be ruled out as a suspect.