A cyclist is taken to hospital in a critical condition after a possible mechanical failure caused him to come off his bike in the Canberra suburb of Deakin this morning.

The man was riding with another cyclist just before 8:00am when he crashed on Kent Street and was taken to the Canberra Hospital with serious head injuries.

Officer in charge of traffic operations, Sergeant Rod Anderson, said roads around the area were closed while paramedics treated the 50-year-old Griffith man.

"He was riding with another fellow, we don't believe there are any other vehicles involved at this stage," he said.

"It appears there might have been some mechanical failure with the bike, and that will form part of our investigation, our crash investigation team will look into all aspects of what might have happened."

The accident was reported on social media, prompting police to call on members of the public to report incidents directly to them, and not on Facebook or Twitter accounts.