"At Heathrow, around 50 per cent of cabin crew have reported as normal and we are therefore increasing the number of long-haul and short-haul flights in our schedule in the days ahead."
At Heathrow, more than 60 per cent of long-haul flights were operating, but only 30 percent of short-haul. At Gatwick, all long-haul flights and more than half short-haul flights were running as normal, as were flights from London City airport, including flights to New York.
However, Unite said that those who had said they would support the strike were staying away from work and a number of planes were starting to "stack up" on the ground at airports.
"105 planes are grounded, right now, and as more and more planes land at Heathrow, legitimately bringing striking crew back, repatriating crew from around the world, those crew are joining our picket lines and those planes are being grounded, Steve Turner, a union official, said.
Al Jazeera's Harry Smith, reporting from outside Heathrow airport, said that it was difficult to gauge how many passengers had been affected by the strike
"Like most news organisations we have not been able to film inside Terminal 5, British Airway's main hub here at Heathrow, from where they send out flights to all over the world," he said.
"We do know that a great many passengers have had there travel arrangements disrupted, however we have seen British Airways' planes arrive and depart.
"We also know that a great many other passengers have got away on flights not marked as British Airways flights. The airline has rebooked them on rival airlines and charted in planes."
The action went ahead on Saturday after last-ditch the previous day collapsed amid accusations from the Unite union that management was not interested in finding an agreement with cabin crew.
The airline wants to save more than $90 million annually through a series of cuts to pay and staffing to help cope with a fall in demand, volatile fuel prices and increased competition from low-cost carriers.
Unite argues it was not properly consulted on the changes.
BA says it had offered to modify the changes, even though they had been approved by Britain's high court, but Unite had declined to put the offer to its members to vote.
Unite has a second four-day walkout planned to begin on March 27 and has said more strikes will be scheduled for after April 14 if the dispute is not resolved.