MADRID: A doctor accused of masterminding a vast doping network that rocked the sporting world and snared top cyclists went on trial in Spain on Monday along with four alleged conspirators.

The case centres on a sophisticated network which was blown wide open on May 23, 2006 when Spanish police seized around 200 bags of blood in an investigation dubbed "Operation Puerto".

Trial witnesses include Alberto Contador, the Tour de France winner in 2007 and 2009 who has returned to competition after a two-year ban for testing positive to banned substance clenbuterol, which he blamed on a contaminated steak.

He was initially linked to the Puerto case but later cleared of any involvement.

The current trial in Madrid will do little to boost the credentials of a sport still reeling from Lance Armstrong's admission that he cheated his way to a record seven Tour de France wins.

The five defendants are facing charges of an "offence against public health", including the suspected mastermind of the network, 57-year-old doctor Eunomian Fuentes.

The other four are Fuentes' sister Yolanda; former Liberty Segura's cycling team director Manolo Saiz; former Comunitat Valencia cycling team chief Vicente Belda along with his deputy, Jose Ignacio Labarta.

Fuentes, in a 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.

They are charged with endangering public health rather than incitement to doping, which was not a crime at the time of the arrests. A Spanish anti-doping law was passed only in November 2006.

The distinction between the two charges is likely to be pivotal.

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

Fuentes, who has denied putting athletes' health at risk, will be the first defendant to take the stand.

In his written defence, a copy of which was published by leading daily El Pais, the Canary Islands doctor said the blood and plasma were stored in "ideal conditions".

"None of the athletes have appeared in this case as having possibly been harmed," he said.

Witnesses such as former cyclist Jesus Manzano, who is scheduled to testify on February 11, will try to refute that assertion.

Since 2004 Manzano, a former rider on Spanish team Kelme, of which Fuentes was then 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).

A case against the network's alleged blood expert, doctor Jose Luis Merino Batres, has been provisionally closed on the grounds that he has Alzheimer's disease.

The investigating judge, Antonio Serrano, closed the case in 2007 and 2008 on the grounds that the doping-related allegations were not illegal at the time and that the small amounts found of blood-booster EPO (erythropoietin) did not constitute a health risk.

The Madrid Provincial Courts obliged him to re-open the case.

The opening hearing in the case was due to deal with procedural matters. The trial is expected to last until March 22.