Hoping to keep as many passengers happy as possible, BA scrambled to rebook some on other services, chartered planes from rival airlines and drafted in volunteer crew. But it still had to scrap more than half of its 1,950 scheduled flights over the period.
Chief Executive Willie Walsh issued a direct apology via YouTube for the walkout, the airline's first in almost 15 years, saying it was a "terrible day for BA."
The strike - backed by some unions in the United States, Germany and Spain - also risked harm to Britain's Labor government before a tough general election expected before June.
On Saturday morning at BA's London Heathrow hub, the cavernous Terminal 5 was nearly deserted after some passengers had flown out early to avoid the strike or simply stayed away altogether. But delays were beginning to mount.
One man trying to get home to Sweden said he was already looking at a four-hour delay.
"Our flight from Dallas arrived two hours late, and now we're waiting to go to Stockholm - that flight is two hours late as well," said Bjorn Barka, a high school principal.
Michael Clements, a security director for a California-based company, was able to check in for his business trip to Amsterdam but was told it would be an hour before he could check in his heavy luggage. "Not enough people," he said with a shrug.
The Eurostar train service between London and continental Europe and Virgin's rail services between London and Scotland were expected to be busy as passengers sought alternate routes.
BA also warned that the disruption would likely last several days beyond the three-day strike, because of a knock-on effect on flights that would carry through to the end of a second strike planned for March 27-30.
"We're in limbo land," said Susan Danby, a school worker from the northern English city of Hull. She is due to fly March 29 to Las Vegas with friends to celebrate their 50th birthdays. "This is our dream trip, we booked it last August and we've been planning it for years."
"We all want more money and better conditions, but people shouldn't ruin other people's holidays," Danby said.
As protesters were readying picket lines Saturday outside London's Heathrow international airport, analysts estimated BA has already lost more than £25 million (more than $37 million) because of canceled tickets and contingency costs.
The two planned strikes combined could cost the airline more than the £63 million ($95 million) that Walsh is trying to save through the disputed changes to workers' pay and conditions.
BA's pilots are not part of the strike, after their union resolved a separate dispute over pay with the airline.
U.S., German and Spanish unions have given some support for Unite's action, but stopped short of pledges for coordinated activity that would disrupt BA's ability to refuel and service the planes it is operating during the walkout.
The U.S. International Brotherhood of Teamsters, which represents 40,000 aviation industry workers, urged travelers to find alternatives and said it was keeping its options "open." The U.S. Association of Professional Flight Attendants also expressed support for BA.
"Many of us have taken decisions not to pull extra flights or routes to help BA pick up the slack," said a pilot for a Chicago-based airline, who spoke on condition of anonymity for fear it could cause him repercussions with his job. "We don't want to be seen as supporting scab labor."
BA said it would handle as many as 49,000 passengers on both Saturday and Sunday. That compares with the average 75,000 for a normal weekend day in March.
At its Heathrow base, more than 60 percent of long-haul flights will operate, but only 30 percent of short-haul. At Gatwick, all long-haul flights and more than half short-haul flights will run as normal. London City flights, including flights to New York, is operating as normal.
Aside from hurting BA financially, the strike is also an unwelcome event for Britain's governing Labor Party before the upcoming national elections.
Prime Minister Gordon Brown irritated Unite, a major political donor, by calling the union "deplorable" and saying as late as Friday night that it should call off the strike.
Britain faces even more possible travel chaos in the run-up to the April 2-5 Easter break, as railway signal workers voted last week to join rail maintenance workers in a strike. The Rail Maritime and Transport union has not called dates for the walkout, but refused to rule out the long Easter weekend.
And over this weekend, engineering works on the London Underground were forcing closures between central London and Heathrow, though the Heathrow Express train service was operating as normal.
Opposition Conservative Party leader David Cameron criticized the stranglehold that unions such as Unite have over the Labor Party.
Cameron is seeking to evoke memories of the difficulties the Labor government, which receives millions of pounds in donations from unions, had in the 1970s, culminating in the mass strikes that became known as Britain's "winter of discontent" and led to the election of Conservative leader Margaret Thatcher in 1979. Unite alone has donated 11 million pounds to the Labor Party in recent years.
"Once again, under Gordon Brown the vested interests triumph and the people lose out," Cameron said Saturday. "This threatens the future of one of Britain's greatest companies along with thousands of jobs."
At a rally of thousands of striking workers in Bedfont, north of London, Unite spokesman Steve Turner said BA "is effectively at war with very proud, very dedicated employees."
Walsh said the disputed changes are critical to the airline's survival - BA has been particularly hard hit by the global economic recession because of its heavy running costs and reliance on increasingly unpopular premium fares.
The airline on Friday offered a compromise on a proposed pay freeze this year, offering a 3 percent rise next year and the year after and then an inflation-linked increase in 2013/14 capped at 4 percent.
Unite argues it was not properly consulted on the changes, which also include a switch to part-time work for 3,000 staff and a reduction in cabin crew sizes from 15 to 14 on long-haul flights from Heathrow.
Any passengers with canceled flights from Saturday through the end of the second planned strike on March 30 will be allowed to rebook on another BA flight within 355 days at no extra charge, but no refunds were being offered, the airline said.
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British Airways Strike Alert
By AP Business Writer Jane Wardell; AP Writer Gregory Katz contributed to this story