Prominent US-based human rights activist Harry Wu has been prevented from entering Hong Kong.

Mr Wu, an American citizen, arrived at Hong Kong's Chep Lap Kok airport from Washington on Sunday night.

He was held overnight in an airport immigration office and deported to Washington via Tokyo on Monday morning, said a Hong Kong-based human rights group.

The incident prompted critics to voice fresh concerns that Hong Kong's freedoms were under threat, and the US consulate said it had sought an official explanation.

"This could have the effect of limiting the freedom of association and the free flow of ideas," said consular spokeswoman Barbara Zigli.

Immigration officials told Mr Wu he was refused entry for "security reasons", according to Frank Lu of the Information Centre for Human Rights and Democracy.

US citizens do not need visas to enter Hong Kong, and it is unusual for them to be refused entry.

'Visiting friends'

Mr Wu, who spent 19 years in Chinese labour camps, is best known for speaking out on human rights.

In 1995, the Chinese authorities arrested him for sneaking in to document prison labour. He was sentenced to 15 years in jail on spying charges and immediately deported to the US.

Mr Wu, 65, had arrived in Hong Kong with a New Zealand journalist, who was sent back to the US on Sunday night.

Mr Wu's wife, Chen Ching-lee, speaking from Washington, said her husband had planned to visit friends in Hong Kong for a few days and had no intention of trying to get into mainland China.

An immigration spokeswoman said: "We do not comment on individual cases."

Mr Wu had previously visited Hong Kong in August 1997, shortly after the territory's handover to Chinese rule, the Information Centre said.

Hong Kong has some autonomy under a "one country, two systems" agreement with China. But democracy activists say Hong Kong's Western-style liberties have been eroded since the handover.

There was anger in February when Hong Kong's Chief Executive Tung Chee-hwa secured a second five-year term in office without facing an election challenge.