George W Bush will express "deep concerns" about the state of human rights in China tomorrow before he arrives in Beijing for the start of the Olympic Games.

The American president will say: "America stands in firm opposition to China's detention of political dissidents, human rights advocates and religious activists. We speak ... 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."

Mr Bush will be speaking tomorrow morning in Thailand on the second leg of his three-nation tour of the region. His speech will be delivered at a convention centre in the capital Bangkok to foreign diplomats, Thai government leaders and business officials before he flies to Beijing.

The text of the speech was released by the White House as Mr Bush flew to Thailand from South Korea where he started his tour.

Today, during a news conference in Seoul with South Korea's president Lee Myung-bak, MrBush said China's pre-Olympics crack down on dissent was "a mistake". "You ought to welcome people being able to express their minds," Mr Bush added.

In tomorrow's speech, he will soften his message by saying any changes in China would have to come, "on [China's] own terms".

But his speech is still likely to enrage China's leaders who are already seething because he met prominent Chinese exiles and dissidents at the White House last week.

In addition, White House press secretary Dana Perino said America would protest against China's decision to deny a visa to former Olympic speed skater Joey Cheek, who was planning to travel to Beijing to urge the Chinese government to help make peace in the war-torn Darfur section of Sudan where more than 200,000 people have been killed and 2.5 million displaced since 2003. China is considered a major influence in the conflict as it controls almost all Sudan's oil potential and also supplies weapons to its government.