The mock scenario was deadly serious: a Vietnamese laboratory had confirmed the bird flu virus had mutated and was spreading from person to person.

Wailing ambulances carried a team of doctors to the affected Hanoi neighborhood. Clad from head to toe in green biohazard suits, boots, goggles and masks, they were ready to battle the start of a pandemic.

The exercise was part of a large-scale bird flu drill in Hanoi Sunday to test the country's pandemic preparedness. Some 900 people — from the military and police to health and agricultural officials — ran through a scenario where throngs of people were falling ill and dying.

Bird flu has already killed at least 68 people in Asia since it emerged in 2003 — nearly two-thirds of them in Vietnam, according to the World Health Organization.

Health experts worry the current virus, which is hard for humans to contract, could mutate into a form that is easily spread among people, sparking a global pandemic that kills millions. So far, most cases have been traced to contact with infected birds.

The alert began after officials participating in the exercise were told lab results showed the deadly H5N1 strain of bird flu virus had changed its genetic makeup and 60 people were all showing typical avian influenza symptoms of high fever and coughing.

The mock Hanoi bird flu pandemic steering committee held urgent meetings. Situation updates were given and tasks were delegated, down to police being directed to manage traffic and public works officials being told garbage must be collected to keep the city hygienic. All the while, people were urged to remain calm and to try to prevent a public panic.

"Right after this meeting, I ask all of you and all branches to deploy urgent measures to prevent the pandemic as planned." Ngo Thi Thanh Hang, vice mayor of Hanoi and committee chair, told the group.

Houses and vehicles were sprayed with disinfectant while doctors quickly took throat swabs from masked patients and prepared to transport the worst cases to hospitals. The area was sealed off and 24-hour checkpoints were set up to ensure no one left or entered the neighborhood.

Health Minister Tran Thi Trung Chien walked through the Viet Hung elementary school, where classrooms had been converted into wards for less-severe patients.

She watched army officials spray the building with disinfectant and saw hordes of doctors scurry from patient to patient armed with oxygen and the anti-viral drug Tamiflu. A makeshift laboratory was also set up inside a military tent to analyze specimens and provide quick test results to determine whether patients were infected with an avian-type influenza.

"Our message is that Vietnam is committed to fight bird flu and a bird flu pandemic if it occurs," Chien said after the 2-1/2-hour practice drill ended. "We call on other countries, together with Vietnam, to do their utmost and find out the most effective measures to fight bird flu if a pandemic occurs."

The World Health Organization has encouraged countries to draft national pandemic preparedness plans, which include simulating mock situations. Australia will carry out a similar field exercise this week.

WHO representatives observed the drill and visited Duc Giang General Hospital, where dummies served as corpses which were sprayed with disinfectant before being placed in body bags.

Peter Horby, a WHO epidemiologist in Hanoi, praised Vietnam's efforts to prepare.

"The real challenge, then, is making sure you can do it without knowing when it's going to occur and anywhere in the country," he said. "That's the tough part."