While Mike Koral was watching the end of Stage 3 of the Colorado Classic bike race, through downtown Denver, he says four motorcycles came onto the closed course and police on foot tried to stop them.

"We were waiting for the bike race on Speer and Zuni to come along," Kural said. "All of sudden, a group of about four motorcycles came up and had cops chasing them and the group of police officers that were on the corner where I was standing came out and tried to stop them because the bike race was coming down."

Koral says he was nervous for the cyclists wondering if the motorcycles would interfere.

"Three of them got around (the officers) and kept going, but a police officer amazingly, he threw his body into one of the motorcyclists and the motorcycle crashed," Koral said. "The motorcyclist got up and tried running away."

He did not get far and officers and as police were placing him under arrest, Koral says, the officer who took him down collapsed in the street.

"The bike dropped, it was loud. Tires were screeching," Koral said. "There was a really intense contact. Imagine a football player with no pads on."

Police confirmed to 9NEWS that both the officer and the rider did not have serious injuries.

A second motorcyclist was taken into custody at 32nd and Federal, but two more remain on the loose.

"The police were awesome," Koral said.