Police arrested a man on suspicion of causing grievous bodily harm on Tuesday after a stabbing near London's British Museum.
The Metropolitan Police said the victim, another man, was knifed in the arm close to the popular tourist attraction. He was taken to hospital and his current condition is unknown.
Visitors were evacuated from the museum, which has since reopened.
The incident, which happened at the junction of Russell Street/Museum St at around 10am, is not being treated as terror-related.
A Met Police statement said: “A man was treated for a stab wound to the arm at the scene and taken by London Ambulance Service to hospital.
“His condition is being assessed.
“This was an isolated incident and there is no outstanding risk to the public. It is not being treated as terror-related.”
People who had been asked to leave the museum said on social media there was a man with a knife in the queue.
A 27-year-old American tourist said she was about to join the queue when she was told to leave by police.
The visitor from New York was leaving a Starbucks directly across from the museum on Great Russell Street when police approached her.
“I was standing across the street at the Starbucks walking out to get into the line,” she said.
“A cop directly in front of us told us we needed to leave and that the crime scene was large.
“I heard that someone was stabbed and the ambulance was parked inside near the grass area and then rushed down the street, right by me, with police following behind.”
A statement from the London attraction said: “The museum was closed this morning due to an incident following a member of the public being attacked nearby.
“The museum's security team supported at the scene until the emergency services arrived. Visitors were evacuated from the museum as a precaution and we wish the victim a full and swift recovery.
“The museum has now reopened with raised security including a heightened search operation.”
George Osborne, former chancellor and chairman of the British Museum, thanked the museum's security team and other staff who reacted promptly to the incident and wished the victim the "best recovery".
The 52-year-old tweeted: "Disturbing news of a knife attack near the gates of the British Museum this am.
"Much thanks to our security team and other BM staff, who reacted quickly, with the police.
"Museum has now reopened; everyone's thoughts at the BM are with the victim and we wish him the best recovery."
The British Museum opened in 1759 and houses a number of controversial objects, including the Elgin Marbles, which British diplomat Lord Elgin removed from the Parthenon temple in Athens in the early 19th century, when Greece was under Ottoman rule.
Another controversial exhibit is the Rosetta Stone, which was discovered in Egypt, and dates back to about 200BC. It was discovered by French troops in 1799 but acquired by Britain under a treaty in 1802.
The only time the stone is known to have left the British Museum was in 1917, during the First World War, when it was moved to a railway tunnel to keep it safe.
The 1963 British Museum Act prevents the institution from giving away objects from its collection except in very limited circumstances.