The British Museum has been partially closed to visitors after a former employee allegedly shut down some of its IT network.
The museum alleged that the contractor entered the building and accessed the systems before being arrested.
It led to some galleries being closed on Friday, the museum added, along with temporary exhibitions that remain closed.
The museum said capacity was limited for this weekend, with members and those who already hold tickets given priority.
A British Museum spokesman told the BBC: "An IT contractor who was dismissed last week trespassed into the museum and shut down several of our systems.
"Police attended and he was arrested at the scene.
"We are working hard to get the museum back to being fully operational but with regret our temporary exhibitions have been closed today and will remain so over the weekend – ticket holders have been alerted and refunds offered."
Customers who already hold tickets for this weekend have been contacted ahead of their visit.
The British Museum - which holds items including the Rosetta Stone, the Anglo-Saxon ship burial at Sutton Hoo and the contested Parthenon Sculptures - sees millions of people go through its doors each year to see the permanent collections and exhibitions.
Last year it was named the top UK visitor attraction, having had 5,820,860 visitors in 2023.
There are currently three temporary exhibitions, including Silk Roads and Picasso: Printmaker.
The museum said it apologised for any inconvenience and asked customers to contact its box office to reschedule their visits or have tickets refunded.
The Metropolitan Police has been contacted for comment.