Coastguard staff are staging the first strike in their history.
Almost half of Britain's 19 rescue centres were closed, the Public and Commercial Services union said.
Up to 700 Maritime and Coastguard Agency employees walked out from 0700 GMT. But staff going to emergencies will not take part, the union said.
The 24-hour strike follows protests about what staff say is "poverty pay". Managers say they are bound by pay constraints.
Centres which closed included Solent, Brixham, Holyhead, Belfast, Forth, Thames and London, the PCS said.
The MCA said contingency plans would be put in place to ensure public safety is not compromised by the action, which did not involve crews of coastguard vessels.
Managers will also be drafted in to answer calls and perform other duties.
Peter Cardy, chief executive of the MCA, insisted Mayday calls, automatic distress calls and 999 calls would still get through despite the strike.
He said: "It is true that the coastguard salaries are lower than those in comparable emergency services - but the work is different."
The union wants coastguards to get another £3,000 on top of their current base pay of £14,000 a year.
STRIKES AT MCA CENTRES
The union's pay co-ordinator Geoff Lewtas told the BBC: "It is a scandal, quite frankly, that an organisation that's got such great importance as the fourth emergency service, is running its operation with its permanent staff paid on shoestring wages.
"It's because they are on poverty pay in many cases, and because their pay isn't on a level with workers in similar situations providing emergency services, such as the police, ambulance and fire."
Mark Serwotka, general secretary of the PCS union, said its members did not take the decision to go on strike lightly.
He said: "There has been a tremendous support for today's strike at picket lines across the country with coastguard members standing up for fair pay.
"They feel let down and betrayed by a refusal to pay them the same as other emergency services and by below inflation pay rises which are cutting their pay in real terms," he said.
The MCA says it is bound by government rules on public sector pay.
Labour MP Gwyn Prosser showed his support for the strike at a protest outside the Dover Coastguard Rescue Co-ordination Centre.
He said: "I've spoken to some people who work in this station who turn in barely £14,000 a year. That's just not fair."