Video footage showing numerous police officers pinning and handcuffing a one-legged black man has sparked anger on social media.
The 11-minute clip, which was uploaded to YouTube, captures five officers straddling and attempting to handcuff the struggling man.
One officer can be seen standing directly on his prosthetic leg.
The man can be heard saying he is in pain and asks to be let go.
His trousers can also be seen falling below his buttocks, where they remain for several minutes before officers pull them back up.
The mobile phone footage was recorded by witness Chaedria LaBouvier on the streets of San Francisco’s financial district.
She said: “This is how you treat citizens? You’re having him partially nude while you pin him down on his prosthetic leg?
“You don’t protect and serve this community if that’s policing to you.”
About 12 officers can be seen forming a wall between the man and a gaggle of onlookers, many with mobile phones in hand, before a wheelchair is brought to him and the clip ends.
The footage emerged amid a national debate over police use of force, particularly against minority groups, following the deaths of a number of unarmed black men at the hands of police.
San Francisco Police spokeswoman Grace Gatpandan said officers encountered the 42-year-old man waving around wooden sticks and detained him for a mental health evaluation as he walked toward street traffic.
Gatpandan said several responding officers knew the man, who had an outstanding felony warrant for aggravated assault.
She said police had not seen any video leading up to the incident, but added Police Chief Greg Suhr and others would “look into this situation thoroughly and determine if there were any violations of department policy.”
She said the case was also sent to the independent Office of Citizen Complaints.
The footage was first published in a posting to the news and features website Medium, and the clip has already generated more than 100,000 views on YouTube.
Medium said the man used the sticks as crutches.
Many reacted to the video on social media, calling the use of force excessive.
“What everyday racist policing looks like: unnecessary force, brutal disrespect and little humanity,” said Twitter user Karen Breakey.
In a previous controversy involving San Francisco police, prosecutors in May dismissed eight criminal cases involving arrests linked to 14 officers who traded racist and homophobic text messages.