BALTANAS, Spain — With the road narrowing and cyclists piling up in front of him, Lance Armstrong tumbled hard off his bike.

He was left with a broken collarbone that will need surgery and questions about whether he’ll be able to contend for an eighth Tour de France title in July.

‘‘I’m alive!’’ he wrote on his Twitter feed. ‘‘Broken clavicle (right). Hurts like hell for now. Surgery in a couple of days. Thanks for all the well wishes.’’

Armstrong will fly back to the United States after being knocked out of the first stage of the Vuelta of Castilla and Leon stage race in Spain.

‘‘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,’’ he said in a statement.

Armstrong is scheduled to compete in the Giro d’Italia from May 9-May 31, then the Tour de France from July 4-26.

‘‘I think for the Giro it’s a very big problem,’’ Armstrong said as he left Valladolid University Hospital. ‘‘For now, the biggest problem is just the pain.’’