Patrick Hutchinson was widely praised after a photo of him helping the man on Saturday went viral.
Describing the events behind the image, he told the BBC the situation "wasn't going to end well" without their help.
"I scooped him up into a fireman's carry and marched him out," he said.
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Speaking about the events leading up to the photo, Mr Hutchinson said he was with his friends when one of them saw an altercation at the top of the stairs by the Southbank Centre, near Waterloo.
"The guy ended up on the floor and these guys [signalling to his friends] rushed in to stop him from getting trampled," said Mr Hutchinson, a personal trainer and grandfather.
"In doing so, they created a barrier around him, and I was the last one to come in. I scooped him up into a fireman's carry and marched him out with the guys around me, protecting me and shielding me and protecting this guy from getting any further punishment."
He said people were still trying to hit the man as they were leaving the scene.
"I wasn't thinking, I was just thinking of a human being on the floor. It wasn't going to end well had we not intervened," Mr Hutchinson said. "I had no other thoughts in my mind apart from getting to safety."
He added: "We did what we had to do. We stopped somebody from being killed."
