A man has been brutally bashed in broad daylight on a packed CBD tram, raising fresh concerns about the safety of Melbourne's trams following alleged sex assaults on the same route.

The man, believed to be in his late 30s or early 40s, was taken to St Vincent's Hospital in a stable condition after the attack on the 96 tram on Bourke Street about 2.20pm on Monday.

The victim with blood spattered on his shirt after a vicious assault on the 96 tram on Bourke Street.

A commuter on the tram said the attack began just before the Queen Street stop.

"I heard this huge commotion and all these people screaming," she said. "I looked up and this man - they were sitting opposite each other - was just punching the (victim) over and over and over, with a huge amount to force.