The 26-year-old rider fell during a descent more than 12 miles from the finish in northern Italy. He lay motionless and bleeding heavily on the roadside before paramedics removed his helmet and tried to resuscitate him.
"He was unconscious with a fracture of the skull base and facial damage," Giro doctor Giovanni Tredici said. After 40 minutes of cardiac massage we had to suspend the resuscitation because there was nothing more we could do."
His body was covered by a sheet and taken away by ambulance about an hour after the accident. Local investigators immediately opened an inquiry. Weylandt's body was taken to a nearby hospital for an autopsy.
Weylandt's father and the cyclist's pregnant girlfriend were en route to Italy and were to be met at Malpensa airport in Milan.
"Things like this shouldn't happen. Absolutely sick to the stomach," British cyclist Mark Cavendish said on Twitter.
Davide Vigano, one of Weylandt's teammates, described the descent as a technical one, with smooth asphalt.
"I wasn't told about what had happened during the race," he said. "I'm lost for words."
Wouter Weylandt (pronounced WOW-tehrk WAY-lahnt) rode for Leopard-Trek. The team put a picture of the smiling rider on its website.
"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," the statement said. "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."
Leopard-Trek general manager Brian Nygaard tweeted that the team would decide later in the day if it would continue to race. The 21-stage Giro finishes in Milan on May 29.
"I'm shocked and saddened. May he rest in peace," Lance Armstrong, the seven-time Tour de France champion who is now retired, said on Twitter.
Organizers canceled the day's prize ceremony. Spain's Angel Vicioso won Monday's 107-mile stage from Reggio Emilia to Rapallo. David Millar of Britain took the pink jersey as overall leader.
Race director Angelo Zomegnan said there will be no afternoon music or festivities at Tuesday's fourth stage. He added that it will be up to the riders to decide how they might want to honor Weylandt.
Almost exactly a year ago - May 10, 2010 - Weylandt won the third stage of the Giro on its final day in the Netherlands.
Weylandt was the first rider killed in a crash in one of cycling's three main tours since Italian rider Fabio Casartelli died during the 1995 Tour de France.
Weylandt is the fourth cyclist to die during Giro and the first in 25 years. The others were Orfeo Ponsin (1952), Juan Manuel Santisteban (1976) and Emilio Ravasio (1986).
At the 2009 Giro, Pedro Horrillo was induced into a coma after falling 200 feet over a guard rail into a ravine during the eighth stage. The following day, the main pack deliberately rode slowly to protest safety conditions.
In 2003, Kazakh rider Andrei Kivilev died after he fell from his bike and fractured his skull while not wearing a helmet during the Paris-Nice stage race. The International Cycling Union subsequently made the wearing of hard helmets compulsory.
Weylandt joined the newly formed Leopard-Trek team at the start of the 2011 season, viewing it as his big break in racing alongside Frank and Andy Schleck and Fabian Cancellara. The move followed six years spent with Quick Step, during which time he rode as a support cyclist for Tom Boonen.
UCI President Pat McQuaid issued a statement offering "sympathies to all members of Weylandt's family, all his friends and teammates, but also to all his colleagues on the Giro, who will have to overcome their grief to continue in the race."
Renato Di Rocco, president of the Italian cycling federation, said Weylandt's death left the sport "in anguish."
"We are all affected by this grief, caused by the imponderable, which is always a risk despite the measures made to insure maximum safety," Di Rocco said. "From that point of view, the organizers did everything possible with great professionalism and timeliness. Faced with a tragedy like this no words are adequate."