BANGKOK, August 6, 2008 (AFP) - The United States opposes China's detention of dissidents and other activists, President George W. Bush will say Thursday in a pointed message on human rights on the eve of the Beijing Olympics.

According to an advance copy of the speech he will give in Thailand before going to China for the opening ceremony of the Games, Bush insisted that the criticisms were not intended to 'antagonise' the Beijing leadership.

But in a message that also expressed optimism about the future of the world's most populous nation, Bush said he had repeatedly underlined America's 'deep concerns over religious freedom and human rights' in China.

'The United States believes the people of China deserve the fundamental liberty that is the natural right of all human beings,' he said in the prepared remarks, to be delivered during his last trip to East Asia as president.

'America stands in firm opposition to China's detention of political dissidents, human rights advocates and religious activists,' Bush 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.'

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

Critics have said that Bush should have boycotted the opening ceremony of the Games because of China's record on human rights, which has been in the international spotlight in the run-up to the August 8-24 Games.

China has meanwhile insisted that the world sporting showpiece should not be politicised, something Bush has previously pledged he would not do.

But the speech lays down his overarching concerns about the Communist-led country -- one of his last high-profile chances to do so as president before the election of his successor in November.

In South Korea on Wednesday, Bush defended his decision to attend the Games -- he said he would cheer on US athletes as well as pay his respects to China -- and in his speech Thursday would say he was 'optimistic' about the future.

'Young people who grow up with the freedom to trade goods will ultimately demand the freedom to trade ideas, especially on an unrestricted Internet,' he said in the advance remarks.

'Change in China will arrive on its own terms and in keeping with its own history and traditions. Yet change will arrive,' Bush said.

'And it will be clear for all to see that those who aspire to speak their conscience and worship their God are no threat to the future of China. They are the people who will make China a great nation in the 21st century.'

Bush meanwhile hailed China's market reforms and economic growth, and said the United States and China shared 'important economic interests' while also cooperating on issues such as the denuclearisation of North Korea.

But he said Beijing was also being given the message that its rising power on the international stage -- something the Chinese leadership hopes to showcase at the Games -- imposed requirements it needed to fulfil.

'We are making clear to China that being a global economic leader carries with it the duty to act responsibly on matters from energy to the environment to development in Africa,' the US president said.