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 25km (15.5 miles) from the finish of the stage from Reggio Emilia to Rapallo.
The 26-year-old lay motionless and bloodied as paramedics tried to revive him using CPR and cardiac massage.
"Despite immediate treatment there was nothing we could do," doctor Giovanni Tredici told Italian television.
"He died from a fracture to the front of his skull. We arrived at the scene 30 seconds after the crash. We tried for 40 minutes to resuscitate him."
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."
An autopsy will be carried out to determine the cause of death.
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."
Reigning Tour de France champion Alberto Contador said: "It's a terrible story and a dark day for the cycling family.
"I want to give all my condolences to the family of Wouter and all his friends and send a message of encouragement and support to the Leopard team and the whole cycling family."
Race director Angelo Zomegnan said he had taken every possible security measure prior to the tragedy but that they would now be beefed up.
"Since the crash, we've gone to great lengths to ensure the security measures already in place are being checked and reinforced by specialist teams," said Zomegnan.
"No race in the world invests so heavily in ensuring we can come to the rescue of crash victims."
Weylandt, whose girlfriend is pregnant, moved to the Leopard-Trek team at the start of the season after turning professional with Quick Step in 2005.
Leopard-Trek general manager Brian Nygaard said: "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 Quick Step team said their "hearts go out to Wouter's family, friends and the colleagues of Team Leopard, in this sad, sad time".
"For all of us, Wouter was a friend before he was a colleague. We remember him as an honest man, always available with a smile on his face and forever generous towards the next guy," read a Quick Step statement.
"Wouter leaves us with a terrible sense of loss and unbearable grief. We want to remember him with arms held high, crazy with joy after a victory, like the one at Middelburg last year.
"This is the image of him that all of us will carry in our hearts forever."
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."
Australian Stuart O'Grady and the Schleck brothers, Andy and Frank, were among his team-mates at Leopard-Trek.
The podium celebration at the end of Monday's stage 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.