The International Olympic Committee has said drug cheats at the London Games "will be caught" but called for critics of the Chinese teen swimmer Ye Shiwen to "get real".
Mark Adams, from the IOC, said the British capital was seeing "the world's best athletes competing at the very highest level" and that records were being broken "all over the place".
He added: "We can't stop speculation. It is inevitably a sad result of the fact that there are people who dope and who cheat.
Olympic Analyst Damon Meheux in the Anti-Doping Laboratory New Frontiers Science Park, Harlow
"But I equally think it's very sad if we can't applaud a great performance. Let's always give the benefit of the doubt to athletes."
His comments came during a row between China and the US over the performance of Ye, who smashed the women's medley 400m in 28.43 seconds. Chinese officials have reacted furiously to remarks by American coach John Leonard, who described the win as "disturbing" and "suspicious".
Culture, Media and Sport Secretary Jeremy Hunt also defended the London Games' record, telling the BBC: "We've been absolutely determined to make sure that this is the cleanest Olympics ever."
Ye Shiwen after winning the 400m women's medley in a world record time
British Olympic Association chairman Lord Moynihan said: "We know how on top of the game Wada [World Anti-Doping Agency] are and Wada have passed her [Ye] as clean. That's the end of the story. And it is regrettable there is so much speculation out there. I don't like it. I think it is wrong.
"That athlete or, indeed, any athlete that has never tested positive is an athlete who should be supported by her federation and, indeed, everybody in the Olympic movement.
"Let us recognise that there is an extraordinary swimmer out there who deserves the recognition of her talent in these Games."
Cleaners and security staff in the Olympic Village have been briefed to report any evidence of doping by athletes, such as needles or discarded medication packets, according to London 2012's head of anti-doping Jonathan Harris.
LONDON 2012 DRUGS TESTING: THE FACTS
1,706 tests have been carried out so far, including 1,344 urine tests and 362 blood tests.
6,250 tests are expected to be carried out
240 substances are banned
One athlete has been suspended, and one was handed a temporary suspension
All medal winners are automatically checked
A £20m drugs-testing facility in Harlow is used by London 2012 to test athletes' blood and urine samples
Around half of all athletes at the Games are expected to be tested