More than 40 athletes taking part in the Beijing Olympics have today signed an open letter addressed to China's government urging it to respect human rights and freedom of religion, particularly in Tibet.

Coming on the eve of the opening ceremony, it marks fresh embarrassment for the host nation, which also faced criticism from George Bush and renewed protests in Tiananmen Square.

Signatories to the letter include the men's 110m hurdles world record holder, Dayron Robles of Cuba, well known to Chinese fans as the main rival to their most famous track athlete, Liu Xiang, the reigning Olympic champion. Others involved included the US 400m runner DeeDee Trotter and the Croatian women's world high jump champion, Blanka Vlasic.

The letter calls on China's president, Hu Jintao, "to protect freedom of expression, freedom of religion and freedom of opinion in your country, including Tibet".

It asks Hu "to ensure that human rights defenders are no longer intimidated or imprisoned" and to end the death penalty.

"China is the focus of worldwide attention," it says. "Your decision on these issues will determine the success the Olympic games and the image the world will have of China in the future."

Chinese authorities are desperate to shift attention away from human rights and on to sporting matters. But this is proving difficult. Today, Bush, making what is likely to be his final speech in Asia as US president, told an audience in Thailand that the US "stands in firm opposition" to political and religious repression in China.

"We press for openness and justice not to impose our beliefs, but to allow the Chinese people to express theirs," he said.

Striking a more conciliatory tone, he added: "Change in China will arrive on its own terms and in keeping with its own history and its own traditions. Yet change will arrive."

A Chinese foreign ministry spokesman, Qin Gang, said the government opposed any use of the human rights issue to interfere in the country's internal affairs.

"The Chinese government puts people first and is dedicated to maintaining and promoting its citizens' basic rights and freedom," he said. "Chinese citizens have freedom of religion. These are indisputable facts."

Bush arrived in Beijing this evening to a cordial greeting from China's foreign minister, Yang Jiechi. He could, however, soon learn if ordinary Chinese have taken offence at his comments when he watches their nation take on the US at both basketball and baseball in the coming days. Bush is also scheduled to have talks with Hu on Sunday.

The Chinese government says human rights have improved, with fewer death sentences, increased numbers of religious worshippers and a temporary relaxation of travel restrictions for foreign journalists.

But the run-up to the games has been marked by protests and calls by foreign leaders for greater openness and respect for civil rights.

As Bush flew into Beijing for the Olympic ceremony, three US Christian demonstrators were dragged away from Tiananmen Square by plainclothes police.

"We come here to speak out against the human rights abuses of the Chinese government," shouted Patrick Mahoney, director of the Christian Defence Coalition, as security officials held up umbrellas and their hands in a vain attempt to prevent the incident being filmed.

"We are the voice of those with no voice. We are the voice of Falun Gong practitioners."

China's government allows religious worship but only within institutions that pledge primary allegiance to Beijing. Police regularly arrest people who choose to worship at underground "home" churches.

Yesterday, two Britons were among four foreigners deported after putting up Free Tibet banners on lamp poles outside the Beijing national stadium.

Most of the demonstrations, however, have been small stunts aimed at foreign journalists rather than attempts to change public opinion inside China. At times, they have blended Monty Python with vandalism, with reporters being sent mysterious messages to pick up keys to empty hotel rooms where they have found effigies left on a bed and slogans daubed on the walls.

Away from the protests the sport has already begun even before tomorrow's opening ceremony. Early rounds of the male and female football tournaments have kicked off, with Chinese fans getting some early cheer as their women's team defeated Sweden 2-1.