(For full coverage of the flu outbreak, click on [nFLU])
* Soccer matches played in empty stadiums
* Mayor says may have to shut down mass transport system
* Empty streets in normally vibrant city (Updates with details from the airport)
By Helen Popper and Mica Rosenberg
MEXICO CITY, April 26 (Reuters) - One of the world's biggest cities slowed to a snail's pace on Sunday as millions in the Mexican capital hid indoors to avoid infection from a flu virus that has killed up to 81 people.
The Roman Catholic faithful listened to mass on the radio rather than go to church, a professional soccer game was played in an empty stadium and weekend cyclists stayed off the road in the normally chaotic city of 20 million people.
"This is the first time I've left the house in two days. I had to get some air," said Juan Casiano, a 39-year-old office worker, as he walked through a park in the Polanco neighborhood. "But I'm going to stay the rest of the day inside."
Fears of a global swine flu epidemic grew with 20 cases in the United States and six in Canada, and possible infections also popped up in Europe and New Zealand. [ID:nN26482522]
All the deaths from the new flu so far have been in Mexico, mostly in and around the capital, one of Latin America's most important centers of business, government, culture and tourism.
Police fanned out across Mexico City's international airport handing out hundreds of surgical face masks and loudspeaker announcements urged anyone with flu-like symptoms to consider changing their travel plans.
"We've been a lot more worried since we got to the airport and you see all these people wearing the masks," said San Francisco resident Cheryl Guerrero, who was leaving Mexico after a holiday.
Doctors examined dozens of scared travelers but no one had yet been prevented from flying, airport officials said.
Bars and nightclubs were ordered shut for 10 days and on Saturday night streets usually packed with revelers in the trendy Condesa district were empty with improvised signs on the darkened windows advising revelers to stay away.
"People last night were buying some wine and beer to drink at home instead of going out," said Ricardo Martinez, who works at a corner convenience store.
Some bars tried to skirt the requirements to let drinkers in, hiring medical personnel to take their temperatures outside and promising to turn away anyone with a fever.
The Mexican tradition of large family dinners on Sunday died hard, with restaurants still serving diners, some of whom wore face masks. Some people maintained their daily routines, jogging or taking walks in the city parks.
Major movie theaters shut their doors and city workers cleaned out ventilation systems in the subway to try to minimize the risk that the swine flu virus might contaminate passengers.
Soldiers gave out surgical masks all around the city.
A professional league soccer game between the popular Pumas and Chivas teams kicked off behind closed doors at the stadium used for the 1968 Olympic Games. Fans were banned under a government recommendation against large crowds gathering.
The giant Aztec Stadium, which has hosted two World Cup finals, was also to be closed to fans when the America and Tecos clubs face each other.
Mass was canceled or curtailed at churches throughout the city, including the cathedral in the massive Zocalo square and the basilica of the Virgin of Guadalupe, where millions of people visit each year to pay homage to Mexico's most revered Catholic figure. Priests were considering putting off wedding ceremonies, said a spokesman for the Catholic archdiocese.
A few worshipers dotted church pews, many keeping their mouths covered and some offering prayers for those already infected with the flu.
"I asked God to prevent more people dying and that this scourge ends quickly," said Oscar Gonzalez, 46, a hairdresser who was on the steps of a church in western Mexico City.
On the doors of the church, signs read: "Under government orders, all religious services have been canceled." Worshipers listened to a broadcast of mass.
Mayor Marcelo Ebrard said his city government was considering the possibility of shutting down the mass transport system.
"If we don't succeed in decreasing the number of people infected we will have to reduce practically all activity in the city," Ebrard said in a radio interview.
The U.S. Embassy in Mexico postponed over 5,000 visa appointments this week to limit the congregation of large crowds. (Additional reporting Rodrigo Martinez; Editing by Eric Beech)