Funeral of Queen Elizabeth II is huge security challenge

Police officers wait outside Buckingham Palace in London. [AP photo]

Street drains and garbage bins are being searched and sealed. On Monday there will be police spotters on rooftops, sniffer dogs on the streets, marine officers on the River Thames and mounted police on horseback.

Flying drones over central London has been temporarily banned, and Heathrow Airport is grounding scores of flights so that aircraft noise does not disturb the funeral service.

Authorities face the challenge of keeping 500 world leaders safe, without ruffling too many diplomatic feathers. Presidents, prime ministers and royalty will gather offsite before being taken by bus to the abbey - though an exception is being made for U.S. President Joe Biden, who is expected to arrive in his armored limousine, known as The Beast.

Another challenge is the sheer size of the crowds expected to gather around Westminster Abbey and along the route the coffin will travel after the funeral, past Buckingham Palace to Hyde Park. From there it will be taken by hearse about 20 miles (32 kilometers) to Windsor, where another 2,000 police officers will be on duty.

The queen is due to be interred in St. George's Chapel at Windsor Castle alongside her husband Prince Philip, who died last year aged 99.

Police are deploying more than 22 miles (36 kilometers) of barriers in central London to control the crowds, and transit bosses are preparing for jam-packed stations, buses and subway trains as 1 million people flood the ceremonial heart of London. Subways will run later than normal and train companies are adding extra services to help get people home.

While many will be mourning the queen, support for the monarchy is far from universal. Police have already drawn criticism for arresting several people who staged peaceful protests during events related to the queen's death and the accession of King Charles III.

Police officers direct mourners outside Buckingham Palace in London. [AP photo]

Cundy said it had been made clear to officers that "people have a right to protest."

"Our response here in London will be proportionate, it will be balanced, and officers will only be taking action where it is absolutely necessary," he said.

Metropolitan Police Commissioner Mark Rowley said the goal was to keep the event safe, "and try to do it in as unobtrusive a way as possible, because this is obviously a solemn occasion."

Dean of Westminster David Hoyle, who will conduct the funeral service in the 900-year-old abbey, said preparations were going smoothly - despite the occasional security-related glitch.

"There was a wonderful moment when I had flower arrangers waiting in the abbey, and no flowers, because, quite properly, the police didn't recognize what the van was and the flowers were sent back," he said.