A man is in a critical condition after suffering a heart attack midway through a Hamilton cycle race.

Waikato police Senior Sergeant Mike Henwood said the man, 73, was cycling along Tauwhare Rd, on the outskirts of Hamilton, in the Winter Fun Ride Series about midday when he suffered the heart attack.

Cordons were eventually put in place and saw cyclists diverted up Bruntwood Rd, along Brinkworth Rd and down Lee Martin Rd.

He is currently in a critical condition, he said.

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Henwood expected the diversions to be lifted soon.

Event organiser Nathan Cox of Dynamo Events also confirmed the cyclist appeared to have suffered from a medical event.

The cyclist, who began riding at 10am, was just a few kilometres from the finish line, he said.

Although the road was closed the remaining cyclists were able to finish the race through the diversion.

They would make a call on whether next week's third of the three-race series would continue either tonight or tomorrow.

The first was held in Ohinewai last weekend and the next due to be start in Cambridge on August 26.