Up to 100 staff members at the National Gallery in central London have gone on strike in a dispute over pay.
Members of the Public and Commercial Services (PCS) union walked out for two hours from midday to protest at their basic pay rate of £7 an hour.
The PCS said the pay, 60p less than the so-called London "living wage", forced workers to take up second jobs.
The National Gallery said the gallery remains open but only 13 out of 66 rooms were open during the strike.
'Angry and betrayed'
General secretary of PCS, Mark Serwotka, said: "Staff who protect important artworks and assist the public are sick and tired of working 50- to 60-hour weeks and having to take second jobs to earn a living wage.
"The refusal by management to reopen pay talks and its imposition of a pay award, just days before Christmas, has left staff feeling angry and betrayed."
Mr Serwotka called for the management to reopen talks and offer a "decent living wage".
A National Gallery statement said the Gallery "cannot afford" to increase staff pay.
"The Gallery, along with other employers, is in a very difficult financial climate and in 2010 will continue to do the best that is possible for staff pay within the constraints of affordability and government public sector pay policy.
"The Gallery has done all it can to keep disruption to the public to a minimum during the industrial action.
"It has, however, only been possible to keep 13 out of 66 Gallery rooms open during the two-hour strike period."
Last May London Mayor Boris Johnson set the London Living Wage at £7.60 an hour, the minimum recommended wage for employees working in the capital.