LONDON: Queen Elizabeth gets four days of celebrations to mark her 60 years on the British throne under way on Saturday with one of her favorite pastimes, a trip to the horse races, as tributes to the long-serving monarch pour in.

Hundreds of thousands of people are expected to descend on London over the next few days for Diamond Jubilee festivities, with millions attending street parties across the country as the nation marks the queen's personal milestone.

"The queen has given incredible service," British Prime Minister David Cameron said.

"She's never put a foot wrong, she's hugely popular and respected here and around the world and it's an opportunity for people to give thanks and to say thank you for the incredible service that she's given."

Across Britain, red, white and blue "Union Jack" flags billow from street lamps, outside buildings, shop fronts and houses, and sales of patriotic souvenirs have rocketed ahead of the celebrations.

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.

As well being head of the Commonwealth of nations mainly made up former British colonies, Elizabeth is also the Supreme Governor of the Church of England.

"I think we've been enormously fortunate in this country to have as our head of state a person who has a real personality - a personality that comes through more and more, I think, in her public utterances," said the Archbishop of Canterbury Rowan Williams, the spiritual head of the Anglican Church.

"Someone with insight and judgement, and with immense stamina and a depth of commitment that I think is immensely impressive to all of us."

The four days of celebrations begin on a fairly low-key note when the queen indulges her long love of horses by attending the Epsom Derby, one of the biggest events in the British horse racing calendar.

On Sunday, there will be a flotilla of 1,000 boats assembled from around the globe traveling 25 miles along the River Thames featuring the queen and her 90-year-old husband Prince Philip on a royal barge, in the largest such pageant for 350 years.

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

The queen's London residence Buckingham Palace will play host to a pop concert on Monday 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 around the Commonwealth.

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. Not all will be cheering for the queen with banner-waving republicans staging a protest at Tower Bridge during the Thames flotilla.

Officials say 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.

Retail groups say 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.

He warned the capital's public transport system and roads would be stretched, but after 18 months of planning, police do not believe there was any major security threat. Attention-seekers posed the greatest problem, Kavanagh commented.

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. Reuters