EXCLUSIVE: Police have been accused of using excessive force during an arrest in Adelaide city.

7News filmed an officer beating two men with his baton, and the incident will now be investigated by the police complaints authority.

Witnesses described it as a sickening attack. With baton drawn, the officer hits one man twice then strikes the other three times.

It was just before 2:30pm in Whitmore Square, and onlookers couldn't believe their eyes.

“He comes out with his baton, pepper-sprayed them, then flogged them, landed the knee on the back, punching him in the head. Cops can't do that mate, you know that's wrong," witness Simon Waye said.

"The attention that they got and the way they were treated was absolutely absurd and disgusting."

Visibly distressed, the men had pepper-spray washed from their eyes as more officers arrived at the scene.

Police say they were questioning the men over alleged drug dealing when they attempted to run off.

It's understood they live at the men's shelter across the road, one is the other's carer.

Locals say while they're used to seeing police in Whitmore Square, they've never seen an arrest quite like this.

"A bit of abuse, a bit of langauge that was about it, that doesn't give them the right to do that to another human mate," witness Michael Reeves said.

Within minutes of being taken to the city watch house, one of the men was rushed to hospital with chest pain.

Both have been charged with hindering and resisting police. One is also accused of assaulting an officer and possessing drug paraphernalia.

The matter will be referred to the police complaints authority.