Police say South Australian Premier Mike Rann was assaulted at a function at the National Wine Centre last night.

They say a 55-year-old man from Burnside in Adelaide has been arrested for allegedly hitting Mr Rann below his right eye during a Labor Party fundraising dinner.

Witnesses say the man made a comment to the Premier and allegedly tried to grab a knife from a nearby table.

Businessman Barrie Harrop helped control the man until police arrived.

"He came in with a rolled up magazine and started beating the Premier across the head five or six times," Mr Harrop said.

"I got up and put the guy into a headlock; he went for a knife on the table and he was begging for me to let him go."

The man has been taken to the City Watch House and is expected to be charged with aggravated assault.