The Team Sky rider was slowing down after the road was blocked due to a huge crash by Orica-GreenEdge's Mitch Docker when he was hit by the motorbike, who had approached the sector at speed.
Viviani was forced into the barriers at the side of the cobbles as fans took evasive action. A spectator managed to catch the clip on camera and uploaded it to Twitter.
Also on the social network, Viviani updated his well-wishers with pictures of the crash and his bandaged-up injuries.
"Today I was unlucky but at the same time the most lucky of the world! A big Chest trauma and two cut (chest and hip)," Viviani wrote on Twitter.
Sky's sports director Servais Knaven said: “If you go in the Arenberg there should not be any motorbikes near the riders. He was in the second group with [Peter] Sagan and [Fabian] Cancellara – at the moment [Viviani] is in hospital. He looks all right, nothing serious we think but you never know."
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