Cuba's Dayron Robles has signed an open letter to the Chinese government
More than 40 Olympic athletes, including Cuban hurdler Dayron Robles, have signed an open letter condemning China's human rights record.
The letter is a Sports for Peace initiative, and was addressed to China's president Hu Jintao.
Other signatories included Croatian world high jump champion Blanka Vlasic and US 400m runner DeeDee Trotter.
The athletes' signatures were collected during a Golden League Athletics meeting in Berlin on 1 June.
Amnesty International in Germany and the International Campaign for Tibet have backed the letter, which appeared in the International Herald Tribune on Wednesday.
It calls on Hu "to protect freedom of expression, freedom of religion and freedom of opinion in your country, including Tibet".
The letter demands that Hu end the death penalty and asks him to "ensure that human rights defenders are no longer intimidated or imprisoned".
"China is the focus of worldwide attention," it concludes. "Your decision on these issues will determine the success the Olympic Games and the image the world will have of China in the future."
The legacy of most elite athletes is to be completely vanilla - the idea that my legacy to this world is putting a ball in a hole is unsatisfactory
A spokesman for Amnesty international in Germany said in a statement that some athletes have chosen to withdraw their support.
"Such retractions take place against a general backdrop of fear in terms of freedom of expression and censorship surrounding the Beijing Olympic Games," it said.
"They also further highlight the failure of the International Olympic Committee to show clear moral leadership ahead of the Games.
"Athletes should feel free to express their personal opinions on human rights without fear or favour."
Britain's former NBA star John Amaechi has backed the Olympians who chose to protest.
"If you are a person whose voice resonates around the world then it is your responsibility to try and effect positive change," Amaechi told the BBC.
The 37-year-old, now Olympic ambassador for Amnesty, demanded that athletes "moved by conscience" speak out.
"To me, it is remarkable that we want to say athletes should shut up and just play," said Amaechi.
"In everyday context they are good enough to tell us what cereals to eat, what shoes to wear, about anti-obesity or whatever, but when it comes to a fundamental tenet of human rights, somehow they are not good enough.
"If we want them to be holistic role models, then let's let them be holistic role models."
Amaechi, who retired from basketball following the 2006 Commonwealth Games, urged competitors in Beijing to make use of their privileged platform.
"The legacy of most elite athletes is to be completely vanilla," he said.
"I played basketball for many years and the idea that my legacy to this world is putting a ball in a hole is unsatisfactory to me.
"If people are satisfied they can play a great forehand volley or make a birdie from a bunker - if that's enough for them, then I cannot comment.
"But it is not enough of a legacy when the opportunity to change the world is in your grasp."
Rule 51 of the Olympic Charter states that athletes are free to voice opinions as long as they do not enter into political demonstrations at events or venues - if they do, they can be disqualified.
Amaechi has become a political activist since retiring from basketball
"It is very convenient to suggest that human rights are political," said Amaechi. "It's simply a way of shutting people up.
"The Universal Charter of Human Rights says that this is not the case and this is something every country of the Olympic community has signed up to."
However, Amaechi stopped short of calling for a boycott on the Games.
"I am not asking athletes to embarrass the Chinese people - they have worked hard. I am not asking for a boycott.
"There have been improvements - I applaud the Chinese government for that and I am not demonising the people of China - but they need support from people who can speak up and perhaps make an impact."
Earlier, China rejected criticism from US President George Bush over its human rights record, and asked athletes to refrain from political demonstration.
"The Chinese government puts people first, and is dedicated to maintaining and promoting its citizens' basic rights and freedom," Foreign Ministry spokesman Qin Gang said.
"We firmly oppose any words or acts that interfere in other countries' internal affairs, using human rights and religion and other issues."
John Amaechi is commentating on the Olympic basketball for BBC TV. Listen to the full interview on BBC Radio 5 Live Sport after 1930 BST on Thursday, 7 August.