The crash happened on a two-lane bridge in Hampton at about 8:30 a.m., just after the start of the 40th annual Granite State Wheelmen Tri-State Seacoast Century ride.
Police said two Massachusetts women were killed. They were identified as Pamela Wells, 60, of South Hamilton, and Elise Bouchard, 52, of Danvers. Two other riders suffered non-life threatening injuries, police said.
Investigators said a 20-year-old motorist was driving southbound and crossed into the northbound lane, hitting the riders. The woman was treated at a hospital for minor injuries and released.
Authorities haven't said whether the driver will be charged. Hampton police Deputy Chief Richard Sawyer said the investigation is ongoing and could take some time while police interview witnesses.
Event organizers didn't immediately return messages seeking comment. A section of Ocean Boulevard, also known as Route 1A, was closed off while police investigated the crash but reopened around 2 p.m., Sawyer said.
Sawyer said riders had to be routed around the bridge closure while police investigated.
The route typically follows the Massachusetts, New Hampshire and Maine coastlines, according to the event's website.
The event included rides of up to 100 miles on Saturday and Sunday. Riders were encouraged to follow all the rules of the road as they encountered narrow colonial New England roads, steel decked bridges and weekend traffic, according to the event's website.
The ride is organized by Granite State Wheelmen, a bicycling club.