LONDON (Reuters) - A flotilla of 1,000 boats will sail London's River Thames on Sunday in a spectacular highlight of four days of nationwide celebrations marking Queen Elizabeth's 60 years on the throne.
Around one million people are expected to line the 7-mile route of the armada accompanying a royal barge carrying the 86-year-old monarch, her husband Prince Philip and leading members of the royal family.
Up and down the country hundreds of thousands more will take to streets adorned in red, white and blue "Union Jack" flags for diamond jubilee parties to honor the second British monarch to mark the milestone.
The only other was her great-great-grandmother Queen Victoria in 1897, and while Britain is no longer a superpower whose empire straddles the globe, surveys show the royal family is enjoying its strongest public support in decades.
The Saturday-to-Tuesday holiday comes just over a year after the wedding of Prince William to Kate Middleton, an extravaganza of pomp and pageantry that led news bulletins the world over and boasted a global audience of up to two billion people.
The queen kicked off the jubilee celebrations with a visit on Saturday to the Epsom Derby in southern England, indulging her life-long passion for horse racing, and a special gun salute boomed from the historic Tower of London.
On Monday the monarch hosts a pop concert outside her London residence Buckingham Palace, where Paul McCartney and Stevie Wonder will be among the acts. Madness are set to take to the roof of the famous landmark to belt out hit song "Our House".
The long weekend concludes with a service of thanksgiving at St. Paul's Cathedral on Tuesday followed by a carriage procession along the broad Mall leading to Buckingham Palace where the queen will wave to the crowds from the balcony.
Not everyone supports the royal family.
The small yet vocal republican movement, which plans a protest during the flotilla, called the jubilee "a celebration of inherited power and privilege, and those celebrations have no place in a modern democracy".
But a poll published last month in the Guardian newspaper showed 69 percent of respondents thought Britain would be worse off without the monarchy against 22 percent saying it would be better off.
"(The republicans) know they can't beat it, the combination of her own sense of duty and hard work and this sort of undisputed mother figure status," royal biographer Robert Lacey told Reuters.
Many Britons are simply indifferent -- 2 million people are leaving the country altogether to make the most of the extended public holiday.
The queen ascended the throne in 1952 on the death of her father King George VI when she was 25 and Winston Churchill was British prime minister.
She has since attended thousands of official functions as the head of state of the United Kingdom and 15 other realms, and is generally seen as a hard-working and uncontroversial figurehead who has been able to adapt to the times.
It has not always been easy. In the late 1990s, the monarch had to guide her family through a slump in popularity over its handling of the death of the hugely popular Princess Diana in a 1997 Paris car crash.
Yet by her Golden Jubilee in 2002 the monarch had regained much of the lost ground, and 10 years later the nation looks ready to throw a party in her name.
For the police, Sunday's flotilla, the largest of its kind since 1662 and the reign of King Charles II, presents a new security challenge.
"We're treating it as a unique event, to have that many dignitaries on that many boats moving along the Thames," London police's Deputy Assistant Commissioner Stephen Kavanagh said.
"We've had officers searching under the water, on the water, in the air and on the land," he told Reuters.
Police have not identified a major security threat, but acknowledge that attention-seeking stunts could disrupt big set-piece events. The weather also threatens to put a dampener on proceedings -- some rain and cool temperatures are forecast.
Reporting by Mike Collett-White; Editing by Ralph Gowling