Mexico shut down schools, museums, libraries and state-run theatres across it overcrowded capital Friday in hopes of containing a swine flu outbreak that authorities say killed at least 20 people -- and perhaps dozens more.

Mexico put the confirmed toll at 20 dead, but 40 other fatalities were being probed, and at least 943 nationwide were sick from the suspected flu, the health department said.

President Felipe Calderon cancelled a trip and met with his cabinet to co-ordinate Mexico's response. The government has 500,000 doses of flu vaccine and planned to administer them to health workers, who are at high risk of infection because of contact with the ill.

Experts are unclear on whether vaccine made to protect against human flu strains would offer any protection against this new swine flu.

There are no vaccines available for the general public in Mexico, and authorities urged people to avoid hospitals unless they had a medical emergency, since hospitals are centres of infection.

They also said Mexicans should refrain from customary greetings such as shaking hands or kissing cheeks, and authorities at Mexico City's international airport were questioning passengers to try to prevent anybody with possible influenza from boarding airplanes and spreading the disease.

"We certainly have 60 deaths that we can't be sure are from the same virus, but it is probable," said Health Secretary Jose Cordova. He called it a "new, different strain ... that originally came from pigs."

Epidemiologists are particularly concerned because the only people killed so far were normally less-vulnerable young people and adults.

Closing the schools across the metropolis of 20 million kept 6.1 million students home from day-care centres through high schools, and thousands more were affected as colleges and universities closed down. Parents scrambled to juggle work and family concerns due to what local media said was the first citywide schools closure since Mexico City's devastating 1985 earthquake.

Authorities also advised capital residents not to go to work if they felt ill, and to wear surgical masks if they had to move through crowds. A wider shutdown -- perhaps including shutting down government offices -- was being considered.

"It is very likely that classes will be suspended for several days," Cordova said. "We will have to evaluate, and let's hope this doesn't happen, the need to restrict activity at workplaces."

Mexico's initial response in its overcrowded capital brought to mind other major outbreaks -- such as when severe acute respiratory syndrome, better known as SARS, hit Asia.

At its peak in 2003, Beijing was the hardest-hit city in the world. Schools, cinemas and restaurants were shuttered to prevent the spread the deadly respiratory virus, and thousands of people were quarantined at home.

In March 2008, Hong Kong ordered more than a half million young students to stay home for two weeks because of a flu outbreak. It was the first such closure in Hong Kong since the SARS outbreak.

Lillian Molina and other teachers at the Montessori's World preschool scrubbed down their empty classrooms with Clorox, soap and Lysol on Friday between fielding calls from worried parents. While the school has had no known cases among its students, Molina supported the government's decision to shutter classes, especially in preschools.

"It's great they are taking precautions," she said. "I think it's a really good idea."