Armstrong will require surgery after fracturing his right collarbone
Cycling legend Lance Armstrong fractured his collarbone in Spain's Vuelta Castilla y Leon on Monday and is a doubt for the Giro d'Italia in May.
The 37-year-old American was caught up in an incident about 12 miles from the finish in Baltanas and was taken to hospital for treatment.
Armstrong will now fly to the US to undergo surgery.
"I am very disappointed. Very," said Armstrong on leaving hospital in Valladolid. "Especially for the Giro."
He later wrote on social networking site Twitter: "I'm alive! Broken clavicle (right). Hurts like hell for now. Surgery in a couple of days. Thanks for all the well wishes."
Armstrong is scheduled to compete in the Giro d'Italia from 9-31 May, with the Tour de France running from 4-26 July.
Astana team leader Johan Bruyneel was optimistic, saying: "It's a fracture without complications and it should be a fast recovery."
It was a shame to lose Lance. We could see that he wanted to use this race as part of his preparations. It was a good chance for us to work together
Armstrong's team-mate Alberto Contador
Television images showed Armstrong falling heavily on day one of the five-day race, before clutching his arm while sitting on the grass.
"It happened just before the end, and everything happened very quickly," said Armstrong.
"Two riders fell ahead of me. I tried to get past and I wasn't able to."
And as he left Valladolid University Hospital, he added: "The collarbone is broken, and I have a little bit of road-rash abrasions.
"I've never had this happen before; it's pretty painful. I feel really miserable."
Astana Team doctor Pedro Celaya confirmed: "Lance suffered a fracture of the middle third of the right collarbone as well as some bruises on his right hip and arm."
Organizers for the Giro d'Italia are holding out slim hopes that Lance Armstrong will still enter their race.
"We're used to watching Armstrong pull off miracles, so we're still hoping he can do one more," said Giro director Angelo Zomegnan.
"We're going to wait until the operation and see what his physical situation is. Sure we're worried." said Zomegnan, who added that he has been in contact with Armstrong's advisers after the crash.
Speaking to the Astana website on Monday, Armstrong added: "In 17 years as a pro I have been lucky to avoid one of the most common cycling injuries.
"The crash has put my upcoming calendar in jeopardy but the most important thing for me right now is to get back home and rest up and begin my rehab."
Armstrong had been racing alongside Astana team-mate Alberto Contador of Spain for the first time, with the pair vying to be team leader.
"It was a shame to lose Lance," said Contador. "We could see that he wanted to use this race as part of his preparations. It was a good chance for us to work together.
Armstrong uncertain about recovery
"Now only I can support him and wish him to recover as soon as possible in order to take the start on the Giro."
The Texan retired in 2005 after winning the Tour de France for a record seventh time, but he announced a shock return last September saying he wanted to raise cancer awareness.
A survivor of testicular cancer, he plans to compete in this year's Tour de France in July after beginning his comeback at the Tour Down Under in Australia in January.