THE 1,500 Vietnamese asylum-seekers evicted from their huts amid waves of tear-gas yesterday will be threatened with relocation to a tough island prison unless they volunteer to return home. They must sign up for the voluntary repatriation programme run by the United Nations High Commissioner for Refugees (UNHCR) or go to Chi Ma Wan detention centre - a prison known as the last stop before deportation and a traditional home to troublemakers. A military-style operation was mounted at Whitehead detention centre in Sha Tin early yesterday, involving 500 officers from the Correctional Services Department and 750 from the Police Tactical Unit, all in full riot gear. The 421 children, 525 women and 510 men were taken to High Island detention centre in Sai Kung. Officers fired 250 canisters of tear-gas to subdue Section Seven of the camp where the most resistance to the move was expected and to dislodge about 200 who climbed on to roofs of huts. A Vietnamese woman fell from the roof and hurt her back. She was in fair condition in Queen Elizabeth Hospital last night. Others suffered scratches and smoke inhalation, but all recovered quickly. Government sources said the group would be transferred to Chi Ma Wan where an upper section houses 86 Vietnamese and the lower section houses 590 Vietnamese illegal immigrants from China. The camp can hold more than 2,000 inmates. Any of the group who chooses to voluntarily return to Vietnam will be sent to a departure centre in the next few weeks rather than to Chi Ma Wan. It is understood the group will be given the weekend to consider whether to join the voluntary repatriation programme. The Security Branch said the raid at the sprawling Whitehead camp was part of a programme to close camps. Commissioner of Correctional Services Eric McCosh defended the heavy security presence, saying order had been breaking down, particular in Section Seven. Boat people there had demonstrated peacefully and staged hunger strikes over the past three months to protest against repatriation, but this had progressed to middle-of-the-night drills, stopping officers making routine searches and blocking entrances tothe camps. It had been decided to move the 1,456 people in Section Seven to High Island to thin out Whitehead's 15,000 population and defuse tensions at the camp. ''Fifteen thousand people in an unstable state is a recipe for disaster,'' Mr McCosh said. ''We have to have sufficient staff to deal with any eventuality, hence the PTU [police tactical unit] and support. ''We spent 28 minutes trying to convince them that packing up their things and moving voluntarily was the best possible action, which they didn't listen to,'' Mr McCosh said in defence of the use of tear-gas. Most of the tear-gas was fired later in the morning when hundreds of inmates refused to come down from the roof. ''We had to use tear smoke, which is the minimum amount of force. How else could we get them off the roof?'' said Mr McCosh. A search of Section Seven after the inmates left turned up 122 home-made weapons, including spears, knives and pokers, and 57 home-made gas masks. United Nations High Commissioner for Refugees chief of mission Jahanshah Assadi said he was not surprised that the Government had chosen to act. ''The ongoing demonstrations have been disrupting normal life in the camps and this is a situation where the Government has chosen to reassert its authority,'' Mr Assadi said. Refugee Concern Hong Kong spokesman Kerry Doyle criticised the tear-gassing which she said put the lives of children at risk. ''Rather than decreasing tensions which is what we needed, the Government has decided to step up their aggressive tactics and now the tensions can only increase,'' Ms Doyle said. She believed that threats of transfer to Chi Ma Wan would be enough to persuade many people to volunteer for repatriation.