"What about his back?" someone asked. The small group decided it was best to risk injury to his spine and begin CPR. We rolled him over. I reckoned he was about 50 and he looked friendly. He was overweight and his teeth were slightly crooked.

Someone took off their T-shirt and put it under the man's head. A smallish guy with glasses positioned himself there and I moved to his chest. The man with glasses put a handkerchief over the injured man's mouth and breathed two heavy breaths. I watched as his chest rose, then started compressions, pushing down hard above his heart. I hesitated when I reached six, trying to recall what I'd been taught at a surf club almost a decade ago. My CPR partner said, "Just keep going."

He told me he'd just done a first aid course and asked if I would like him to take over. Of course, I said, relieved to hand over responsibility to someone better trained. I noticed the injured man was wearing a wedding ring. The guy in glasses kept up CPR as we waited for an ambulance. One man was on the phone to emergency and relaying instructions, another was checking for a pulse. Everyone was remarkably calm. A couple of cyclists were slowing down and waving through other riders. This little group of strangers had become a team.

At last, we heard a siren and shortly afterwards a female paramedic climbed over the guard rail with her equipment. I estimated it had been about 10 minutes since I'd arrived - it may have been less. I felt the injured man's hands - they were cold. The man with glasses continued CPR until the paramedic took over. After he stood up I put my arm around his shoulder and squeezed him. He was pale and frightened and a bit sweaty from the effort. He adjusted his glasses and said, "I think he's dead."

We looked around for the injured man's backpack but he appeared to be carrying only an iPhone attached to a pair of earphones. By now, a second ambulance had arrived and an officer cut off the man's shirt and attached the pads of a defibrillator to his chest. I noticed he had a long scar on his chest and wondered if it was from a heart operation.