A registered Level 2 sex offender is facing a new charge after allegedly grabbing a female victim on Tremont Street near Downtown Crossing and Boston Common, officials said.

Dorchester man Kenneth Osborne, 65, has been charged with indecent assault and battery on a person over 14.

Last Friday afternoon, bystanders flagged down Boston police officers to report an indecent assault near 178 Tremont St.

The visibly distraught alleged victim told police that she was on her phone when an unknown man, later identified as Osborne, pressed against her and grabbed her buttocks.

She told police that Osborne fled toward Boylston Street when she turned around and asked, “Do I know you?”

She tried to follow Osborne while filming him with her phone, but she was unable to keep up and lost sight of him.

Responding officers broadcasted Osborne’s description to other police in the area.

About 30 minutes later, cops found and detained Osborne in Downtown Crossing. Officers conducted a field identification with the victim, who told them, “That’s the man I was chasing.”

Osborne was convicted of assault to rape and indecent assault and battery in 2006, and sentenced to state prison.

“While it’s fortunate that this victim didn’t sustain physical injuries, no one should downplay the fear and personal violation she experienced, nor the impact that incidents like this have on our sense of neighborhood safety and quality of life,” Suffolk DA Kevin Hayden said in a statement.

Judge Richard Sinnott ordered Osborne held on $5,000 bail with GPS monitoring if released.

Osborne’s next court date is March 5 for a pre-trial hearing.