Workers at a number of UK museums, galleries and heritage sites have begun strike action in a row over jobs, pay and pensions.
Half of the 66 rooms at London's National Gallery were closed on Thursday, while one floor of the National Portrait Gallery was closed.
Tate Liverpool's galleries were closed to the public completely.
More stoppages will follow on Friday as a result of walkouts planned by the Public and Commercial Services union.
The National Portrait Gallery apologised to customers in a statement, saying it was "necessary for some gallery rooms to be closed" due to the strike action.
Workers at the Natural History Museum and Victoria and Albert Museum in London are expected to participate in the strike later, though both attractions will remain open.
In Liverpool the Merseyside Maritime Museum, the International Slavery Museum and three others will be closed to the public from Friday lunchtime to Saturday afternoon.
The Museum of Liverpool will be closed for part of Saturday but will reopen once the 24-hour action is completed.
Employees at English Heritage sites, including Stonehenge in Wiltshire, will take action on Sunday.
The industrial action is part of a three-month campaign of industrial action prompted by a long-running dispute over jobs, pay and pensions.
The PCS union, the UK's largest civil service union, is planning a national walkout at the end of June.