BBC correspondent Jim Fish reports on Clinton's arrival in Beijing

Mr Clinton said he was disturbed by the allegations, and ordered his ambassador to China, James Sasser, to lodge an official complaint.

China responded by dismissing the reports as rumours, and saying China was opposed to any foreign interference in its internal affairs.

But one of Mr Clinton's aides, the National Security adviser, Sandy Berger, immediately rejected the Chinese statement, calling it unsatisfactory and thoroughly unacceptable.

Sandy Berger: "This is not surprising but it is not acceptable"

"People are not debris to be swept up for a visitor," Mr Berger said.

"If China is going to be a nation whose practices are fully acceptable to the international community then this is not a step in that direction," he said.

Mr Berger said that Mr Clinton would raise the issue when he meets the Chinese president, Jiang Zemin, on Saturday.

Robin Murro from Human Rights Watch: "There have been several detentions in recent weeks"

As Mr Clinton left the ancient capital of Xian for Beijing, two of three activists reported to have been detained were freed. A Hong Kong human rights group said one dissident remained in police custody.

Correspondents point out that the Chinese denial does not necessarily cover the detention of dissidents - as opposed to their arrest - which follows a different legal procedure.

Visit to the Terracotta Army

Mr Clinton, who has now arrived in Beijing, began his first full day of a nine-day visit to China at Xia He, a village near Xian, observing a local democracy scheme at work.

The Clinton family visited the excavation site of the Terracotta Warriors in Xian

Later, Mr Clinton visited one of the country's most popular tourist sites in the region - the imperial tomb and the Terracotta Warriors.

The Terracotta Warriors are an army of more than 7,000 pottery soldiers and horses, created and buried 2,200 years ago to guard the tomb of Emperor Qin Shi Huangdi. They were unearthed beginning in 1974.

Mr Clinton will stand at the edge of Tiananmen Square in Beijing for a ceremony at the Great Hall of the People where he will be offcially welcomed to the capital of the People's Republic of China by President Jiang Zemin.

BBC China specialist James Miles: President Clinton must have expected this

Following his visit to Tiananmen Square, Mr Clinton will be the guest at a state dinner at the Great Hall of the People, the seat of China's Congress.

He and Jiang Zemin are also expected to meet privately before holding a news conference.

Controversy over Tiananmen visit

Bill Clinton is the first US president to visit China since the Chinese army crushed pro-democracy demonstrations centred on the square in June 1989.

The square, which has room for a million people, remains the focus point for discussions on China's record on human rights and freedom of speech

A diverse group of organisations has urged that he should not visit Tiananmen, contending that his presence would dishonour the hundreds of people killed by army tanks and guns.

Despite announcing that he intended to raise human rights issues during the visit, many Americans oppose his trip, saying the ceremony in Tiananmen Square would help to whitewash human rights abuses by Beijing.

A further day of sight-seeing has been scheduled for the Clintons after the Beijing leg of the trip on Sunday.

Then Mr Clinton will travel to Shanghai on Monday for three days of meetings and speeches.