Beijing - Two activists from Australia and the US and two journalists were detained briefly in China today after the unveiling of a banner in a Beijing park reading No Olympics for China until Tibet is Free, witnesses said.

The activists, Liam Phelan, 33, of Newcastle, and American colleague Han Shan, had unveiled the 3m banner in a park celebrating China's ethnic minorities. "They have been detained and had their passports taken away," Paul Bourke of the Australia Tibet Council told AFP earlier in the day.

"This is precisely what we are concerned about - no expression of political opinion is allowed in China. We are concerned about them and we're doing all we can to find out what has happened to them."

Two journalists, including a reporter for Britain's Guardian newspaper, were also detained, the International Tibet Support Network said, but were later released.

The two activists were then released later in the day, although they were not given back their passports, Alison Reynolds, director of the Free Tibet Campaign, told AFP.

The two activists unveiled the red-and-yellow banner on a bridge at the park near the site to be used for the Beijing Olympics in 2008. It was on display for 10 minutes before police moved in, witnesses said.

Beijing occupied Tibet, which it insists has been an integral part of the Chinese nation for centuries, in 1951.

Since then, it has been accused of trying to wipe out Tibet's unique Buddhist-based culture through political and religious repression as well as mass ethnic Chinese immigration.

Rights groups have begun stepping up their campaigns against China as the clock starts ticking to 2008 and the first ever Olympics to be held in China.