Museum workers are striking over pay, at some of the country's most popular tourist attractions.
The walkout affects science museums in London and Swindon, the National Media Museum in Bradford, and the National Railway Museum in York.
The museums say 110 out of more than 1,000 staff across all sites are on strike, but unions say only a "handful" have crossed picket lines.
The venues say they will remain open despite the protests.
Unions also said staff were "furious" after being given a below-inflation pay deal, adding that the starting rate for employees in the disputed museums is £11,342.
London's Science Museum said about 64 of its staff were believed to be on strike, out of 530 employees.
However, the Public and Commercial Services (PCS) said that only 15 staff members had gone into the London museum, with only a "handful" of others elsewhere around the country crossing picket lines.
A spokesman said: "The majority of those going in are managers, who are staffing the galleries after only an hour's training.
"The figures about strike numbers are totally disingenuous."
The PCS said the pay deal on offer to staff at the museums had been delayed by more than a year and would mean many people would receive less than 3% for 2007-08 and 2.7% for 2008-09.
"Our pay awards compare very favourably with those in the wider public sector"
Mark Serwotka, general secretary of the PCS, said: "Strike action is not a step our members take lightly but, faced with pitiful pay levels and real-term pay cuts, hard-working staff have taken the decision to make a stand.
"Museum staff along with other civil and public servants have grown increasingly frustrated with the government's policy of below-inflation pay, which is driving down the living standards of a diverse and dedicated workforce.
"Management have to recognise that it has to pay a fair wage and the government needs to rethink its draconian pay policy."
'Very disappointed'
The Prospect union said the increase was "insulting" and only set aside 1.5% for a cost-of-living increase in 2007.
It also said the only pay increases allocated to 2008 were performance-related.
Prospect national officer Emily Boase said: "Members are furious and their patience has been exhausted."
The National Museums of Science and Industry group (NMSI), which runs the affected sites, said its pay awards compared "very favourably" with those in the public sector.
A spokeswoman said: "We are very disappointed that negotiations have broken down despite our very best efforts and flexibility on the options available and in phasing in the changes over a period of time. We remain open to further negotiations.
"Our pay awards compare very favourably with those in the wider public sector. We benchmark our pay rates and we know that staff are paid competitively.
"Across all our sites we have a workforce well in excess of 1,000 people. The majority are not involved in the dispute - only around 15% voted to strike and it is not clear whether they all will take action."
About 200 workers at the Museum of London and the organisation's Archaeology Services went on strike on Monday in protest at a two-year 2% pay rise.
The museum said it was bound by public sector pay policy set by the government. Its pay deals differ from the NSMI's because it has a different pay structure.
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