Police believe the man fell off his road bike on Kent Street after his bike malfunctioned. He was treated by paramedics at the scene before he was taken to Canberra Hospital in a critical condition.

"He [lost a lot of blood] at the scene and he has some fairly serious head injuries," Officer in charge of traffic operations Rod Anderson said.

Police said he was wearing a helmet and do not believe he was riding recklessly.

"There is nothing to suggest he was doing anything other than riding along as most Canberrans do and unfortunately this is a sad incident," Officer Anderson said.

The man fell off his road bike on Kent Street and was treated by paramedics at the scene. He has been taken to Canberra Hospital in a critical condition.

Police were notified about the accident at about 7.45am.

Police said the man was cycling with another man when he came off his bike, no other vehicles were involved and he was the only one to crash.

Police believe the man's bike may be to blame.

"He was riding with another fellow and we believe it might have been a mechanical problem with the bike, but that will form part of our investigations," Officer Anderson said.

Police said passersby were quick to lend a hand.

"The fellow who was riding with him rendered assistance, and a number of other cyclists and vehicles stopped and helped out with first aid," he said.

Kent Street, between Denison Street and Strickland Crescent, was closed while paramedics treated the man.

The road has now reopened.

Motorists are advised to avoid the area.

Witnesses or anyone with information is urged to contact Crime Stoppers on 1800 333 000 or via act.crimestoppers.com.au.