They added that the suspect ran for about half a block with the mother and several bystanders in pursuit.
A well-known Australian disc jockey, Roscoe Bradley Holyoake, 34, from Perth, was arrested over the incident.
"Shortly after we walked by her, we heard her screaming, yelling for police, for help, saying that someone had taken her child", Walker said.
A Perth DJ accused of trying to snatch a two-year-old boy from his mother in a San Francisco street could face a decade behind bars if convicted.
She said she struggled with him and freed the boy from his grasp.
Sabrina Walker said she's proud of her courageous husband.
Mr Walker, a dentist, said he and other bystanders chased after Holyoake on foot.
"I think my fatherly instincts kicked in and I did what I thought was right", Adam Walker told KTVU. "And he didn't fight back, didn't struggle, just obeyed my commands, walked to the sidewalk and put his hands on the wall and got on his knees".
Australia media reports Holyoake is a popular DJ known as DJ Roski. In addition to regular club night appearances and performances surrounding Sydney Mardi Gras, he has also volunteered for community radio station RTRFM.
In December past year Holyoake was a performer at Tim Minchin's wrap party for his new mini-series "upright", an experience he described as an "absolute pleasure", 9 News reported.
Online jail records showed that Holyoake was booked Friday about 9 p.m. and was still in custody on Monday on $500,000 bail on a kidnapping charge.