Officials say the 48-hour action could cause disruption at the country's ports and airports, with Heathrow Airport, Europe's busiest, expecting the strike to affect services at border control on Monday morning.
It will also hit courts, ports, jobcentres, benefit offices, tax centres and emergency police call centres.
Heathrow said passengers should expect some additional delay going through immigration.
Mark Serwotka, general secretary of the PCS, said there was "incredible anger among hard-working public servants who are seeing their terms ripped up".
"The strike will show how vital these people are to the running of our society. Those on strike today deliver services that touch our everyday lives from the cradle to the grave," the Press Association quoted him as saying.
More strikes could follow as the union hopes the proximity of an election will force the government to scale back changes to redundancy pay.
The action reflects frayed union relations with the ruling Labour government.
Labour, which relies on union funding, is facing the prospect of defeat to the opposition Conservatives at a poll expected on May 6, after 13 years in power.
However, opinion polls have shown the Conservatives' once considerable lead dwindling this year, raising the possibility of a hung parliament.
Whoever wins the election will have to make substantial cuts to some government department budgets in order to bring down a budget deficit forecast to soar
above 12 per cent of gross domestic product this year.
The Conservatives, regarded as less union-friendly than Labour, have indicated they will focus most of the necessary fiscal tightening on spending cuts rather than tax hikes and plan to freeze public sector pay for most workers in 2011.