Resident Peter Fox-Worthington was returning home with his wife when he came across the accident.
He ran to the man's aid as others stood and looked on.
Fox-Worthington, who is the youth leader of Cambridge St John, said the man's bike was removed from between his legs before himself and a woman began administering first aid.
Shortly afterwards, emergency services arrived at the scene and took over first aid.
He said it appeared the cyclist had been unconcious for several minutes before they arrived and began doing compressions.
A man following the cyclist in a car confirmed he hadn't been hit by any other vehicle before he suddenly fell.
He was now concerned for his health and hoped he would pull through.
The Herald understands the man suffered a medical event while riding.