Spain - 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.

Police busted the Spanish network in 2006 when they seized 200 bags of blood and other evidence of performance-enhancing transfusions, in an investigation dubbed “Operation Puerto”.

Among the five defendants charged with an “offence against public health”, the most prominent is the suspected mastermind of the network, 57-year-old doctor Eufemiano Fuentes.

Investigators listed 58 cyclists suspected in the scandal. Fuentes told Le Monde newspaper he had provided services to others, including tennis players and footballers but later retracted that claim.

Of the 58, six have received sporting sanctions: Spanish rider Alejandro Valverde, Germans Jan Ullrich and Joerg Jaksche and Italians Ivan Basso, Michele Scarponi and Giampaolo Caruso, who was later cleared by the Court of Arbitration for Sport.

Fuentes's lawyer Julian Perez-Templado told AFP after Monday's hearing that the doctor would not reveal any more names of people for whom he carried out transfusions.

The other four defendants in the trial are the doctor's sister Yolanda; former Liberty Seguros cycling team director Manolo Saiz; former Comunitat Valencia team chief Vicente Belda and his deputy, Jose Ignacio Labarta.

Fuentes, in a dark suit and blue tie, and the other four defendants were swarmed by reporters as they arrived at the Madrid court for the start of the hearing, but made no comment.

Perez-Templado said Monday's hearing dealt with procedural matters and certain demands by civil parties in the case. Judge Maria Santamaria adjourned the trial until Tuesday morning, when Fuentes was due to be the first to testify.

The defendants are charged with endangering public health rather than incitement to doping, which was not a crime at the time of the arrests.

The prosecutor, seeking a two-year prison sentence plus a two-year professional ban for the accused, will have to show that the blood transfusions put the riders' health at risk.

Fuentes has denied that charge but witnesses such as former cyclist Jesus Manzano, scheduled to testify on February 11, will challenge his claim.

Manzano, a former rider on Spanish team Kelme of which Fuentes was the head doctor, has alleged generalised doping in the team and says he himself underwent transfusions of adulterated blood.

The 30-year-old Contador, due to appear on February 5, was cleared of any involvement in the Puerto affair by a Spanish judge and the sport's world governing body the International Cycling Union (UCI).

In a separate case, Contador was later banned for two years after testing positive for the prohibited substance clenbuterol, which he blamed on a contaminated steak. The trial is scheduled to last until March 22.

Investigating judge Antonio Serrano had closed the Puerto case, arguing that the alleged acts of doping were not illegal at the time and that the small amounts of blood-booster EPO found did not constitute a health risk.

The Madrid provincial courts obliged him to re-open the case.

In London, the UCI announced it had disbanded an independent commission it had set up to investigate alleged involvement by the global governing body in the Lance Armstrong doping scandal.

UCI president Pat McQuaid said they had been left with little choice after World Anti-Doping Agency (WADA) labelled it a “useless exercise” and the United States Anti-Doping Agency (USADA) refused to cooperate.