He was immediately kicked out of the Tour and suspended by his team, Liquigas.
"When are these idiots going to learn that it's over?" said Pat McQuaid, the leader of the International Cycling Union. "They continue to think that they can beat the system. They're wrong. The system is catching up all the time."
The 37-year-old Spanish rider, who rode alongside Lance Armstrong for three years as one of U.S. Postal's cyclists, was taken away by French police within two hours of the positive test. He can ask for a "B" sample, although these rarely clear riders. If he does ask for it and he fails that test, too, he will be fired by the team, its directors said.
Beltran was targeted by the French anti-doping agency (AFLD) before the race had even started, after blood tests carried out on July 3-4 showed "abnormal parameters", the AFLD's chief Pierre Bordry told The Associated Press.
"Yes, they were of a nature to target him, that was why he was tested on Saturday" July 5, Bordry said. "The parameters were abnormal.
"There are not just traces of EPO, there is EPO," Bordry said. "Whether there is a lot or a little, EPO is forbidden."
Beltran might not be alone, either, with Bordry adding that other riders
- so far unidentified - are also under the spotlight for having abnormal blood parameters.
Liquigas spokesman Paolo Barbieri said French police had taken Beltran from the team hotel to the town of Aurillac where Friday's seventh stage finished. He confirmed police were searching rooms at the team's hotel.
Before the positive test, an exciting seventh stage had seen Luis-Leon Sanchez of Spain win with a solo ride and Kim Kirchen of Luxembourg retain the yellow jersey.
Kirchen still leads Australia's Cadel Evans by six seconds, with Monday's first big mountain stage approaching fast on Bastille Day, the celebration of the French revolution that saw the aristocracy overthrown and heads roll.
Heads certainly turned Friday with the news of the Tour's first doping test, which now means four ex-Armstrong teammates have tested positive in the past four years.
In addition to Beltran, Floyd Landis, Roberto Heras of Spain and Tyler Hamilton
- all former Postal riders during Armstrong's seven Tour wins from 1999-2005
- failed doping tests after quitting the Texan's team.
Though Beltran will not continue in the Tour, his team will.
Last year, Cofidis withdrew after Cristian Moreni of Italy tested positive for testosterone, and pre-race favorite Alexandre Vinokourov's Kazakh-owned Astana team was kicked out by race organizer ASO after he tested positive for blood doping.
Depending on what Beltran tells the police, Liquigas may eventually be forced out, too.