Services including job centres and courts are being hit and thousands of schools closed as public sector workers across the UK stage a 24-hour strike.
Picket lines have been set up as the Public and Commercial Services Union (PCS) and three teaching unions protest at planned changes to their pensions.
They say the plans mean more work and contributions for a reduced pension.
The government says the plans are "fair to taxpayers" and other trade unions are continuing with negotiations.
As well as pickets, union leaders and activists will hold a march and rally in central London, where police leave has been cancelled.
Airport delays
The impact of the co-ordinated industrial action began to be felt at ports and airports on Wednesday evening, when some UK Border Agency staff walked out from 1800 BST.
Travellers have been warned to expect delays on arrival at UK ports and airports.
http://www.bbc.co.uk/news/uk-13967580
Note: Update Record, 30 June 2011 Last updated at 08:26 GMT