A central London avenue leading to Buckingham Palace was closed in a security alert on Monday on the eve of a visit to Ireland by Britain's Queen Elizabeth and a week before U.S. President Barack Obama visits Britain.
In a separate incident, police carried out a controlled explosion to destroy a suspicious bag in a central London street, although it turned out to be harmless, a police spokesman said.
The spokesman said there was an "ongoing security alert" in central London but refused to say what had prompted it or whether it was connected to next week's state visit by Obama.
"There are security checks going on in and around the Mall," he said. The Mall is the broad avenue leading to Buckingham Palace, the residence of Queen Elizabeth, who goes to Dublin on Tuesday for the first visit by a British monarch since King George V in 1911.
Militant groups opposed to British control of Northern Ireland are the biggest risk during the queen's four-day visit, which will be surrounded by intense security.
A 1998 deal ending Irish nationalists' guerrilla war against British rule of Northern Ireland set the clock ticking on a royal visit but it was not until police and justice powers were transferred from London to Belfast last year, the last piece of devolution, that the diplomatic wheels could be set in motion.