The 37-year-old American legend was injured when he fell along with several other riders about 20km from the finish line of the first stage of the Tour of Castilla y Leon in central Spain on Monday.

''I'm alive!'' Armstrong wrote on his Twitter feed after leaving hospital. ''Broken clavicle [right]. Hurts like hell for now. Surgery in a couple of days. Thanks for all the well wishes.''

Armstrong is now likely to be off the road ''three to four weeks,'' according to Vuelta of Castilla and Leon doctor Alberto Gomez.

Armstrong admitted the injury put his participation in the Giro d'Italia in May is in doubt.

''For the Giro, it will be very complicated,'' Armstrong said.

It's also likely to have an impact on his grand ambition to win an eighth Tour de France in July as it sets him back in his duel with Alberto Contador for leadership of their powerful Astana team in that event.

Astana manager Johan Bruyneel, a good friend of Armstrong's, always said that the road would decide the team's leader. And the road may well have decided.

Initially, Astana's heavyweights were only set to race against each other at the Tour de France the most prestigious race in the world in July.

That is why it seems ironic that the most decisive moment in Armstrong's return to racing should have happened at a minor race, which was initially only set to free the team of the pressure of fielding the two men together before the Tour.

Bruyneel said Armstrong's injury was ''a clean fracture without complications''. AFP