Cycling authorities say 11 riders were injured, one of them seriously, in a pile-up during a race at Sunday morning's Cyclothon on the Tsim Sha Tsui waterfront.

One cyclist in the men's open race lost control of his bike near Salisbury Road Garden, causing several others to go to ground. Race officials briefly suspended the event before allowing it to resume.

The Hospital Authority said one rider was in a serious condition and eight others were stable in hospital.

One rider said the accident was a scary sight.

"When some riders cut my teammate off, my teammate was squeezed out and bumped into a cone. He was fine. But when he collided with the third cone in a row, he fell and swiped on the left side of the road. I heard 'bang, bang, bang', as cyclists behind him collided one after another and fell like dominoes," he said.

"It is a straight downhill road. You cannot hit the brakes there. You can only crash into each other ... especially when many cyclists are riding at high speed. We had no way to escape."

The chairman of the Cycling Association, Leung Hung-tak, said the accident was not a result of problems with the design of the course, noting that a similar route had been used in the past.

He said the spot where the cyclists collided does have room for multiple riders to pass through safely.

Leung also defended a decision not to immediately suspend the race when the crash happened, adding the judges were following instructions from the international cycling governing body UCI.

"Actually, the cyclists slowed down when they passed through the area because after the accident happened, our staff and judges were there to give directions. But later we felt the need to step up the management and so we suspended the race," he said.

Leung said accidents can happen at any top cycling race and the incident will not affect the city's events in future.

The crash took place during the City Cycling Challenge, part of a series of competitions for club cyclists in Greater Bay Area cities. Unlike the longer Cyclothon events covering longer distances, it involves a multi-lap race along a 3.7 kilometre circuit between Hung Hom and Tsim Sha Tsui East.

Thousands of riders enjoyed the earlier 50km and 30km events, which started in Tsim Sha Tsui and covered a wide area of Kowloon and crossed the bridge to Tsing Yi.