Two men have been arrested after another man was allegedly coward punched and knocked unconscious at the Adelaide Oval footbridge on Saturday night.

SA Police alleged two 21-year-old men approached a 38-year-old man and a 37-year-old man at 10:30pm on the footbridge and a verbal altercation followed.

Thousands of football fans were in the city to watch the Crows play GWS at Adelaide Oval that evening.

The 38-year-old man was allegedly punched before his head struck the ground and he fell unconscious. 

He was taken to hospital with non-life-threatening injuries. 

The other victim, a 37-year-old man, was also allegedly assaulted.

Police said witnesses alerted officers and followed the alleged offenders in the city. They were seen entering a licensed venue on King William Street.

The men allegedly swapped clothing while inside the venue.

The pair, both aged 21 and from Mount Barker, were arrested. One was charged with assault and assault causing harm while the other was charged with assisting an offender.

Police have asked for any witnesses who have not yet spoken with SAPOL to contact Crime Stoppers.