London bomb threat: a bomb threat made by an Irish extremist group forced London police to seal off the mall leading up to Buckingham Palace.
"A bomb threat ... has been received relating to central London today. The threat is not specific in relation to location or time," London police said in a statement.
A security source said the caller had used a codeword known to the police, lending credibility to the threat.
The warning came on the eve of the first visit to Ireland by a British monarch in a century and a week before U.S. President Barack Obama makes a state visit to London.
Police said earlier that a security alert had led to the closure of the Mall, a broad avenue leading to Buckingham Palace, Queen Elizabeth's residence, but refused to say what had prompted it.
A Northern Irish republican militant group, the Real IRA, told Queen Elizabeth, head of state of Ireland's former colonial master, last month she was not welcome on Irish soil.