A woman who was walking on 5th Street near Market was stabbed by another woman in an apparently unprovoked attack on Wednesday, a witness told The Chronicle.

Valentin Wong, 37, said he saw one woman appear to punch another woman as both were walking toward one another at around 5:15 p.m.

“It looked like a regular punch, but apparently she was stabbed,” Wong said. “There was a lot of blood. It was vicious.”

Police confirmed it was a stabbing.

Police rushed to the area to aid the victim, who had a wound in her collarbone area and was sitting against a window of the Nordstrom at 5th and Market. She was taken to a hospital by ambulance conscious and alert. Her condition was unknown.

Wong said the suspect was arguing with a man as they walked along 5th Street and the victim was walking with friends — and as the two groups were about to pass one another, the stabbing occurred with “one single strike.”

Wong said the suspect kept walking, yelling words that he could not understand. The victim sat on the brick sidewalk as witnesses crowded around, asking if she was OK.

“It was senseless,” Wong said.

Wong and other witnesses helped identify the suspect, who was apprehended just down the street on Market.

San Francisco police public information officer Grace Gatpandan said police are investigating the circumstances leading up to the attack. Gatpandan did not release the name of the suspect.

Police cleared he scene just before 6 p.m. and dropped bleach on a pool of blood on the sidewalk.

Lauren Hernandez is a San Francisco Chronicle staff writer. Email lauren.hernandez@sfchronicle.com Twitter: @LaurenPorFavor