(Reuters) - Britain embarks on four days of pomp, pageantry and patriotism on Saturday to mark Queen Elizabeth's 60th year on the throne, with the monarchy's popularity surging and celebrations bringing cheer to a nation struggling in harsh economic times.

"Union Jack" flags fluttered from buildings, shops and train stations across the country, thousands of street parties have been planned and huge crowds are expected to flock to Diamond Jubilee festivities in a country emblazoned red, white and blue.

To royalists, the occasion is a chance to express their thanks and appreciation to the 86-year-old Elizabeth, head of state for 16 countries from Australia and Canada to tiny Tuvalu in the Pacific Ocean, for her years of public service.

For others, the chance of some extra days off work and to enjoy the sort of extravaganza and public ceremony for which Britain is renowned has made it a welcome break from austere times, pay freezes and deep public spending cuts.

Republicans hope the occasion marks the last hurrah of a dying anachronism, while some 2 million people are leaving Britain altogether to go on holiday.

"Original jubilees were invented in the 19th century by the popular press as modes of national celebration for which the monarchy and monarch was almost incidental," said royal biographer Robert Lacey.

He said the jubilee was as much about society celebrating itself as it was about the head of state and the now largely symbolic institution of the monarchy.

"They tend to work best in times of economic hardship. It provides a tonic for the country," Lacey told Reuters.

Having acceded to the throne in February 1952 on the death of her father George VI when Winston Churchill was prime minister, Elizabeth is now the longest-lived British monarch.

Only her great-great-grandmother Victoria spent longer on the British throne and she looks on course to overhaul her as longest-serving monarch in 2015.

During her reign there have been 12 British prime ministers, 12 U.S. presidents, six popes and she has visited 116 countries.

THAMES FLOTILLA

The four days of celebrations begin on a fairly low-key note on Saturday when the queen indulges her love of horse racing by attending the Epsom Derby.

The following day will witness what organizers hope will be a spectacular flotilla travelling 25 miles along the River Thames featuring 1,000 boats assembled from around the globe with the queen and her 90-year-old husband Prince Philip on a royal barge, the largest such pageant for 350 years.

Thousands of street parties are also planned across Britain on Sunday, including one on Downing Street outside Prime Minister David Cameron's office, as part of a "Big Jubilee Lunch".

The queen's London residence Buckingham Palace will play host to a pop concert featuring the likes of Paul McCartney and Elton John, before a network of 4,200 beacons will also be lit across Britain with more set alight in the Commonwealth of mostly former British colonies of which Elizabeth is the head.

The celebrations culminate on Tuesday with a memorial service at St Paul's Cathedral, a carriage procession through central London and flypast by present and former royal air force aircraft.

Huge crowds are expected for the events with estimates that about a million people will travel to London on Sunday alone, and police have warned the capital's public transport system and roads will be stretched.

THOUSANDS OF STREET PARTIES

There are some 9,500 street parties planned in England Wales and ABTA, the British travel association, said almost 2.5 million Britons were expected to take part.

London's Heathrow airport said some 780,000 people were due to arrive in the next few days although ABTA said an estimated 2 million Britons were planning to head overseas to take advantage of the two extra public holidays.

Patriotic products featuring the Union Jack flag have been flying off the shelves, and Britons are expected to spend 823 million pounds ($1.28 billion), nearly double what they paid out on last year's royal wedding of the queen's grandson Prince William and Kate Middleton.

Supermarket Tesco, the world's number three retailer, expected to sell 2.86 million flags by the end of the weekend, while rival Sainsbury's said it had sold 252 miles of bunting, enough to decorate the entire length of the Thames.

But rather than a boon, the Bank of England and economists warn the extra public holidays will hit growth in the second quarter, bad news for an economy that has slipped back into recession and where growth remains elusive.

"It is likely that there will be a significant hit to GDP in the second quarter, which will be partly recouped in the third quarter," said Howard Archer, economist at IHS Global Insight.

Last year's royal wedding and the extra public holiday that attracted was cited as one of the special factors that knocked up to 0.5 percent off GDP growth in the second quarter of 2011.

Police said the weekend would include the largest royal security operation ever conducted. Some 13,000 officials including about 6,000 police officers will be on duty for the Thames pageant, which poses challenges never before encountered.

"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 told Reuters.

After 18 months of planning, police do not believe there is any major threat, but attention-seekers posed the greatest problem with the celebrations attracting global media attention.

In April, a protester disrupted the annual Cambridge versus Oxford university rowing race on the Thames by swimming into the path of the crews and police chiefs admit they cannot guarantee similar embarrassments would not occur.

"There is no plan along that length of river, with that number of people on both sides of the Thames that can prevent anything happening," Kavanagh said.

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