Two London police officers suffered minor injuries to their arms while detaining a man on The Mall outside Buckingham Palace, Metropolitan Police said.
Authorities said the man is being held on suspicion of assault of police and grievous bodily harm.
A statement from New Scotland Yard said the officers stopped the man around 8:35 p.m. (3:35 p.m. ET) because they believed he was in possession of a knife.
The officers were treated at the scene. No one else was reported injured, police said.
Buckingham Palace is not on "lockdown," Metropolitan Police said. The incident is not being treated as a terrorist attack, but the suspect is being questioned, a spokesperson said.
Queen Elizabeth II was not at the palace, her official residence, palace officials said. She is at Balmoral Castle, her holiday residence in Scotland.