THE crunch of marching boots and nervous chatter from the small army advancing on Hong Kong's most notorious detention centre breaks an eerie stillness over Tolo Channel and Ma On Shan. A bobbing sea of helmets, providing cover for more than 1,000 faces etched with apprehension at the task ahead, is moments away from gate-crashing the dreams of the sleeping Vietnamese boat people. Wave after wave of police and prison officers with riot shields, guns, tear-gas and batons demonstrate an extraordinary show of prepared force. Two police armoured personnel carriers, readied to quell feared rioting with an arsenal of noxious gas, form a rearguard. The spotlight from a slowly circling helicopter picks out barbed wire, steel and wretched huts. ''My god, it's like something out of an Oliver Stone movie. Who do they imagine they're facing?'' said a worker with the United Nations High Commissioner for Refugees, her normal access to the camp suspended ''for operational reasons''. By 6 am, Whitehead is bathed in the first light of dawn and the strategists are gearing up for a pre-emptive strike. The objective? ''To move 1,500 VMs [Vietnamese migrants] from Section 7 to High Island Detention Centre. Don't ask me why, it was a Security Branch decision,'' said an official from the Correctional Services Department. Minutes later the opening salvoes, 20 tear-gas canisters, are launched on Section 7, reputed to hold the ''hard core'' of Vietnamese likely to put up the most dogged resistance. The gas is blinding. It stings every centimetre of exposed moist skin, wracks nostrils, chokes lungs and makes stomachs heave. Outside, a handful of sunglass-wearing police open the throttles of their Hondas and speed to the gate command post to await the men, women and children who opted to submit for herding on to caged buses. Their worldly belongings, mostly clothes stuffed in red, white and blue carry bags, leave the camp as vans bearing choi sum and morning newspapers arrive. At 8.20, the Hondas move out to form an escort for the first convoy of two buses and two vans transporting the Vietnamese to High Island camp. Weeping, wailing mothers and blankly-staring children press faces against the mesh and peer into a hundred camera lenses. Others appear untroubled, curious at the attention. As the procession of buses continue, agency workers waiting outside the camp speculate about what is happening to the asylum seekers they are helping. ''The Vietnamese have been bracing themselves for this, they've expected it for weeks,'' says one agency worker. ''Their biggest fear is the tear-gas and most of them have fashioned masks from cloth and glass. We didn't expect them to give in without a fight.'' The fight, albeit one-sided, is resumed shortly after 10. A series of muffled detonations signals a heavier gas attack on hundreds of Vietnamese who have defiantly climbed on to roofs and are frantically waving flags, with messages of SOS. Trails of white smoke arc over the steel huts as canisters home in from many directions. Clouds of searing chemical drift across the huddled, fidgeting figures. They bury their heads in their hands or try to make use of their makeshift masks. Some vainlyfan the approaching plume. The smoke clears to reveal the squatting stay-puts. There's another attack. Clusters of canisters bursting in air form a thicker, more potent persuader. Then nothing. ''All the VMs on the roof have climbed down voluntarily after counselling by the CSD staff,'' the Correctional Services Department official told the press a short while later. ''There are no casualties,'' he adds, as an ambulance with several people in the back left the camp, its lights flashing. And in the midst of everything, a young couple picnicking on the rocks beside the high fence wait for bites on the fishing lines cast in the sun-dappled water.