TUC general secretary Brendan Barber said the public sector was "under attack" by the government, adding that the action was justified.
"With the scale of change the government are trying to force through, making people work much, much longer and get much, much less, that's the call people have made," he said.
Public and Commercial Services (PCS) union members who work for the UK Border Agency have gone on strike but airport sources suggested to the BBC that immigration controls were at two thirds of normal staffing levels - more than the 30-50% predicted previously.
More than 1,000 demonstrations were expected across the UKThe Immigration Services Union says 80% to 90% of staff are striking, with 22 out of 23 workers at Calais port not showing up for work and, as far as they are aware, none of their members working at Heathrow.
A UK Border Agency spokesperson said: "Early signs show our contingency plans are minimising the impact of strike action, but waiting times at some ports may still be slightly longer than normal."
A Downing Street spokesman said: "I think what it shows is that our contingency plans have been reasonably effective in mitigating the impact of the strikes.
"Obviously, there has been an impact on schools and we always knew that there would be a significant impact on schools.
"We are confident that all the essential services have been maintained throughout the day. A lot of non-urgent work was rescheduled."
Simon Walker, of the Institute of Directors, told the BBC News Channel the strike was doing "significant damage" to the economy.
"If you're damaging the productive capacity of this country you're really doing huge damage to the fabric of the economy and that will last a long time and impact on all of us," he said.
