British police arrested a man carrying a knife as he attempted to enter Buckingham Palace, the official London residence of Queen Elizabeth II.
The queen was not at the palace at the time.
Scotland Yard said the 44-year-old man "attempted to enter Buckingham Palace via the north centre gate but was apprehended immediately by police."
"The man was searched and found to be in possession of a knife, which was seized," the force said in a statement.
The incident happened at around 11:30 am (11.30pm NZT), it said.
The man remains in custody on suspicion of trespassing on a protected site and possession of an offensive weapon, the police said.
A Buckingham Palace spokeswoman told AFP: "I can confirm that the queen was not in residence when the incident happened but other than that we are not commenting."
The arrest comes after a series of security incidents at Buckingham Palace, the main London residence of the royal family.
In September a man who scaled a fence to get into the palace was arrested on suspicion of burglary.
A few days afterwards, with security on high alert, jittery royal protection officers challenged the queen's third child Prince Andrew in the gardens of Buckingham Palace.
The police later apologised to the Duke of York, who is fifth in line to the British throne, but denied a newspaper report that officers had pointed guns at him and shouted at him to get down on the ground.
In February police used a Taser stun gun to subdue a man waving large knives outside the palace.
The most serious breach at the palace came in 1982 when unemployed man Michael Fagan got inside the queen's private chambers while she was in bed.
Fagan spent 10 minutes talking to the sovereign after climbing over the palace walls and up a drainpipe, and she raised the alarm when Fagan asked for a cigarette.