British fan Guy W captured the footage as the riders entered the Arenberg sector, with huge crash causing the peloton to come to a halt.
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Viviani looked to be off the back of the group and was slowing down when he was hit from behind by the motorbike, to the horror of the crowd watching on.
Fortunately for the Italian only his leg was trapped under the fallen bike, although he was sent into the metal barriers at the side of the road with some force.
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Sky sports director Servais Knaven told reporters that Viviani was taken to hospital nearby.
“We lost Viviani in Arenberg so hopefully they make something out of it because he was hit by a motorbike,” he 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 he is in hospital. He looks all right, nothing serious we think but you never know.
“Just the way it happened, some spectator told me then he told me myself so I think that’s a bad thing. They hit Elia or someone else and he landed in the fences.”
The incident comes just two weeks after Wanty-Groupe Gobert’s Antoine Démoitie died after being struck by a motorcycle at Ghent-Wevelgem.
A Trek-Segafredo rider who was near Viviani can be heard on the video asking why the motorbike rider was so close to the riders going onto the dangerous cobbled sector at high speed.