US President George W Bush has expressed "deep concerns" over China's human rights record in a speech on the eve of the Beijing Olympics, BBC reported.
"The US believes the people of China deserve the fundamental liberty that is the natural right of all human beings," he said in the Thai capital Bangkok.
He praised China's economy but said only respect for human rights would let it realise its full potential.
Mr Bush has been criticised by some campaigners for going to the Games.
He was due to fly to Beijing following the speech in Bangkok, a stop on his final trip to Asia before he leaves office in January.
The wide-ranging address, which included criticism of the regime in Burma, was more nuanced than Mr Bush's past speeches on China, the BBC's Jonathan Head reports from Bangkok.
It is unlikely to cause much offense in China, our correspondent says, and many people will see it more as a valedictory speech for Mr Bush's record in Asia rather than an outline of future US policy.
President Bush said he was optimistic about China's future and said change in China would arrive "on its own terms".
But his criticisms of China's human rights record were clear.
"America stands in firm opposition to China's detention of political dissidents, human rights advocates and religious activists," he said.
"We speak out for a free press, freedom of assembly and labour rights not to antagonise China's leaders but because trusting its people with greater freedom is the only way for China to develop its full potential.
"And we press for openness and justice not to impose our beliefs, but to allow the Chinese people to express theirs."
Beijing police dragged away three US Christians who tried to demonstrate on Tiananmen Square on Thursday in support of religious freedom.
A plainclothes policeman was amongst the security officials who stepped in and led Reverend Patrick Mahoney, director of the Christian Defence Coalition in Washington, away from the centre of the square.
Four pro-Tibet activists from Britain and the US were arrested and held briefly in Beijing on Wednesday after a protest close to the Olympic stadium.
The US, Mr Bush said, recognised that the growth sparked by China's free market reforms was "good for the Chinese people" and the country's' purchasing power was "good for the world".
On foreign policy, he commended China's "critical leadership role" in the negotiations to end North Korea's nuclear weapons programme, and the "constructive relationship" between Beijing and Washington over Taiwan.
But without going into detail, he added that China as a "global economic leader" had the duty to "act responsibly on matters from energy to the environment to development in Africa".
"Ultimately," said Mr Bush, "only China can decide what course it will follow."
"Change in China will arrive on its own terms and in keeping with its own history and traditions."
While "optimistic about China's future", he added:
"Young people who grow up with the freedom to trade goods will ultimately demand the freedom to trade ideas, especially on an unrestricted internet."
This was an apparent reference to Chinese restrictions on certain news and human rights websites.
Beijing recently appeared to ease its curbs after foreign journalists covering the Olympic Games in China complained about being denied access.