Museums in Europe are seeing initial high demand as they open their doors following months of closure because of the coronavirus pandemic.

The Louvre in Paris, the world’s most-visited museum, sold out of the 7,400 tickets available when it reopened Monday after almost four months. However, this was just a fraction of its usual occupancy. Before the outbreak of Covid-19, the museum typically welcomed between 30,000 and 40,000 visitors a day, mostly from outside France.

Since it closed on March 13, the Louvre has lost 40 million euros ($45 million) in revenue, according to Nadia Refsi, head of the museum’s press division. At best, it expects to bring in 20% to 30% less revenue than in the previous summer due to social-distancing restrictions and reduced tourism, she said.

London’s National Gallery is set to reopen on Wednesday after being shut an unprecedented 111 days. It will be the first national museum in the U.K. to welcome visitors following the country’s lockdown.

The gallery, which previously accepted walk-ins, has instituted an advanced reservation system online, although entry for the permanent collection remains free. All available tickets for its first week are sold out, as are those for the first month of a special exhibition of Italian painter Titian’s works, according to the museum’s website.

Museums are intensifying cleaning procedures, requiring visitors to wear face masks and instituting one-way systems to enforce social distancing. Some, including the Vatican Museums, are testing the temperatures of visitors upon entry.