Cabinet Office minister Francis Maude has called the PCS' decision to strike "futile, counter-productive and irresponsible."
He added that the decision to strike benefits no one, but "threatens the services people rely on."
He said: "I can reassure the public that the Government has done everything we can to minimise disruption [today]. Rigorous contingency planning is in place to try and limit the impact of the strike action and to ensure that affected public services operate as effectively as possible."
More top news
Hefty showers and longer, slow moving downpours into this evening and again on Saturday - some lively with thunder.
We are entering - and we have voted to enter - a period of intense political and economic uncertainty.
David Cameron has announced he is to resign as prime minister by October. We look at who could replace him.