A man has been charged with multiple offences after he allegedly attacked a busker in Adelaide's Rundle Mall with the performer's own ukulele.

The victim, who was dressed as a pencil when the incident occurred, was allegedly bashed in the head with the instrument about 12:00pm.

He received non-life-threatening injuries, police said.

Members of the public held down his alleged attacker until officers arrived.

"I just witnessed a street performer in Adelaide's Rundle Mall have a member of the public reach forward, grab his ukulele and smash him over the head with it," a witness tweeted.

"I was rather enjoying his jokes as I walked past and hope he's okay.

"Props to the brave men [who] leapt to action to restrain the assailant."

Police charged the 29-year-old man from Clearview with offences including theft, property damage, assault causing harm, carrying an offensive weapon and possessing a controlled drug.

He is expected to appear in the Adelaide Magistrates Court on Thursday.

They have asked witnesses who have not yet spoken to officers to contact Crime Stoppers on 1800 333 000.

"Police would like to thank the members of the public for their assistance," SA Police said in a statement.

"Anti-social behaviour or violence of any kind will not be tolerated."

The Adelaide Fringe Festival began on Friday, and the city is currently playing host to hundreds of performers from across the country and around the world.