Mick McCarthy from the PCS defends his members' actions in London
Up to 200,000 civil servants are holding a two-day strike over plans to cap redundancy pay, the Public and Commercial Services Union has said.
It says workers could lose a third of their entitlement over cuts under the civil service compensation scheme.
Members work in areas such as the courts, ports, job and tax centres, and some services have been affected.
The government estimated that only 81,000 union members were striking and said the changes were fair.
Cabinet Office minister Tessa Jowell said services to the public were largely unaffected and most civil servants were working normally - including a majority of PCS members.
Under the new system - which takes effect in April and will save about £500m - anyone earning £30,000 or less will be entitled to a maximum of three years' pay or £60,000, whichever is lower.
Redundancy is currently calculated on length of service, with a month's pay for every year worked.
Those earning £30,000 or more will be paid a maximum of two years' pay.
The PCS, Britain's fifth biggest union, said an employee with 20 years' service earning £24,000 a year could lose £20,000 as a result of new caps.
Tessa Jowell said the terms were better than in many private firms
But the government said those earning £30,000 or less - 80% of all staff - would still get up to between two and three years' salary.
The PCS union warned queues would build at ports and airports, court cases would be delayed, and driving tests may be cancelled.
The Courts Service said no courts were closed as a result of the action, but added there had been some reduction in services.
Its striking members include staff at government departments, as well as workers in Parliament, museums and the Royal Courts of Justice.
At Westminster, the BBC's Emma North said a small picket had gathered outside Parliament and there were other protests spread across 500 sites in the capital.
Julie Goodman, a security officer at the Palace of Westminster, told the BBC: "The government is making attacks on my terms and conditions and I can't let that happen without saying something about it."
A protest is planned in the city centre on Tuesday.
Contracts 'ripped up'
BBC correspondent Mark Simpson, reporting from a small picket line outside a tax office in Belfast, said there was no indication of huge public support.
He said the PCS expected 2,000 people at a rally in the city centre later on Monday.
The walkout, which includes 30,000 union members in Scotland, is the biggest show of industrial unrest in the civil service since 1987.
Tessa Jowell: "I urge PCS members...to go back to work"
In Scotland, up to 30,000 union members are estimated to be taking part in the action.
It includes 200 of the Scottish Parliament's 500 staff, a Holyrood spokeswoman said.
Civil servants at job centres also joined in, but PCS leaders admitted they were disappointed with the general turnout.
They claimed their members were on such low wages, many simply could not afford to strike.
Local union representative and sheriff clerk deputy Steve Scott, 52, said the proposed cut "could be illegal".
He said: "It is part of our contract that we get a good payoff if we are sacked. Losing that is just making it cheaper for the government to sack us."
Bernard Harkins, the PCS union's national branch secretary, said members have been asked to refuse to work overtime all week and added there could be another strike next week.
More action is planned ahead of the general election nationwide.
PCS general secretary Mark Serwotka said the government had ripped up already low-paid workers' contractual entitlements to redundancy pay, meaning they could be sacked on the cheap.
He told BBC Radio 4's Today programme: "It's very destabilising for our members, some of whom have worked for over 30 years for the public service, to suddenly realise they could lose their job and actually a large amount of money they would have depended on.
"People over the years have accepted that pay isn't what it should be but they felt their job was secure and their pension was decent.
"Now in the last few years, 100,000 jobs have gone, and many more are likely to go after the election, so people feel very vulnerable."
Ms Jowell said 85% of civil servants and 70% of PCS members were working normally and the "fair" redundancy package had been agreed by five out of six unions - the GMB, Prospect, Unite, FDA, and Prison Officers' Association.
She said: "Across the country services to the public are largely unaffected - all job centres and benefits offices are open, border entry points are working normally and court services are being maintained."
Earlier, she said fewer than one in five members voted in favour of strike action - about 10% of the total civil service workforce.
"We have made extraordinary efforts to make sure the lowest paid are protected," she added.
The proposals brought the civil service in line with the rest of the public sector, she added.
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