Belgian cyclist Wouter Weylandt has died after crashing during Monday's third stage of the Giro d'Italia.
The Leopard-Trek rider fell at high speed during a descent about 20km (12.4 miles) from the finish of the stage from Reggio Emilia to Rapallo.
The 26-year-old lay motionless on the roadside as paramedics tried to resuscitate him.
"Despite immediate treatment there was nothing we could do," doctor Giovanni Tredici told Italian television.
"Weylandt was the victim of a fall and we are still trying to reconstruct the dynamics of what happened.
"We tried for 40 minutes to resuscitate him."
The crash had left Weylandt bloodied and unconscious and requiring cardiac massage.
He received emergency medical treatment by race doctors and was scheduled to be airlifted to hospital but had to wait as an emergency helicopter looked for a suitable landing spot.
Leopard Trek spokesman Tim Vanderjeugd said: "I can confirm the death of Wouter Weylandt.
"He was taken to hospital but had already died at the scene."
Isle of Man cyclist Mark Cavendish said on Twitter: "Things like this shouldn't happen. Absolutely sick to the stomach. My thoughts are with his family. RIP Wouter Weylandt."
Seven-time Tour de France champion Lance Armstrong said on Twitter: "I'm shocked and saddened. May he rest in peace."
Team Sky rider Russell Downing, who is also taking part in the race, told Twitter: "after seeing that i feel sick inside...Rip wouter...u will be sadly missed. lost for words."
Weylandt moved to the Leopard-Trek team at the start of the season after turning professional with Quick Step in 2005.
WOUTER WEYLANDT FACTS
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Born: 27/09/84, Ghent
First professional victory: GP Briek Schotte, 2005
September 2008: Wins 17th stage of the Vuelta a Espana from Zamora to Valladolid
May 2010: Wins third stage of Giro d'Italia from Amsterdam to Middelburg in Holland.
2011: Joins Leopard-Trek from Quick Step
9 May: Dies, aged 26, after crash on third stage of Giro d'Italia
Among his team-mates are Australian Stuart O'Grady and the Schleck brothers Andy and Frank.
A further statement from Leopard Trek general manager Brian Nygaard read: "Today, our team-mate and friend Wouter Weylandt passed away after a crash on the third stage of the Giro d'Italia.
"The team is left in a state of shock and sadness and we send all our thoughts and deepest condolences to the family and friends of Wouter.
"This is a difficult day for cycling and for our team, and we should all seek support and strength in the people close to us."
The International Cycling Union added in a statement: "The UCI has heard of Wouter Weylandt's death with great sadness and president Pat McQuaid's thoughts and prayers go to the rider's family and friends, but also to the riders, who will overcame the shock to start to race again tomorrow."
The podium celebration at the end of the race was cancelled by organisers.
Weylandt was the first rider killed in a crash in one of cycling's three main tours since Italian rider Fabio Casartelli in the 1995 Tour de France.
He is the fourth cyclist to die during the Giro and the first in 25 years. Orfeo Ponsin died in 1952, Juan Manuel Santisteban in 1976 and Emilio Ravasio in 1986.
