A doctor accused of masterminding a vast doping network that snared dozens of cyclists has gone on trial in Spain along with four alleged conspirators.

Trial witnesses include Alberto Contador, the Tour de France winner in 2007 and 2009, who returned to competition last year after a two-year ban related to a separate case in which he denied doping.

The trial in Madrid will do little to boost the credibility of a sport reeling from US rider Lance Armstrong's admission that he doped his way to a record seven Tour de France wins.