UPDATE 2-UK Queen at races to start anniversary celebration
* Queen at Epsom Derby greeted by cheering crowds
* Races launch four-day Diamond Jubilee celebration
* Thousands of street parties planned
EPSOM, England, June 2 (Reuters) - A beaming Queen Elizabeth arrived at the races on Saturday to indulge a lifelong passion for horses and launch four days of nationwide Diamond Jubilee celebrations marking her 60 years on the British throne.
Wearing a blue coat and matching hat on a chilly summer's day, the 86-year-old was greeted by tens of thousands of flag-waving well-wishers at the Epsom Derby in southern England to watch one of the racing calendar's richest events.
Welsh opera singer Katherine Jenkins led the crowd in singing the national anthem as the queen and husband Prince Philip looked on, reflecting a mood of patriotism that has swept the country as the queen reaches a rare milestone.
Only one monarch has celebrated a diamond jubilee before - Queen Elizabeth's great-great-grandmother Victoria in 1897.
"I would have stood there in the pouring rain to have sung that, it was such an honour," said Jenkins.
"This is definitely a day I will never forget. There's such a sense of celebration in the air, everybody's really proud and happy and, I think, inspired by this."
The queen is a keen horse rider and breeder, and visits Epsom every year. While few personal details are known about the famously discreet head of state, it is reported that she reads the Racing Post sports newspaper over breakfast each morning.
Once the day at the races is over the focus turns to London, where huge crowds are expected to line the streets and the River Thames for a series of spectacular events, although rain is forecast which could dampen enthusiasm.
Millions more are expected to attend street parties across the country as surveys of public opinion show the royal family's popularity is as high as it has been for decades.
Across Britain, red, white and blue "Union Jack" flags billow from street lamps, shop fronts and houses, and sales of patriotic souvenirs have rocketed ahead of the celebrations.
On Sunday, a flotilla of 1,000 boats assembled from around the globe will travel along the Thames to accompany the queen and her 90-year-old husband on a royal barge, the largest such pageant for 350 years.
Thousands of street parties are planned across the country, including one in Downing Street outside Prime Minister David Cameron's office, as part of a "Big Jubilee Lunch".
The queen's London residence Buckingham Palace will host a pop concert on Monday featuring the likes of Paul McCartney and Elton John, before a network of beacons are lit across Britain and around the Commonwealth.
The celebrations culminate on Tuesday with a memorial service at St Paul's Cathedral, a carriage procession through central London and a fly-past by present and former Royal Air Force aircraft.
To royalists, the jubilee is a chance to express their appreciation of a woman who acceded to the throne in February 1952 on the death of her father George VI, and is the oldest British monarch.
For others, the chance of some extra days off work and to enjoy the sort of public ceremony for which Britain is renowned has made the jubilee a welcome break from austere times, pay freezes and public spending cuts.
Republicans hope the occasion marks the last hurrah of what they say is a dying anachronism they estimate costs the taxpayer up to 200 million pounds ($307 million) a year.
About a million people are expected to travel to London for the flotilla alone, but not all will be cheering for the queen - banner-waving republicans plan to protest at Tower Bridge during the flotilla.
Police said the weekend would include the largest-ever royal security operation. About 13,000 officials including around 6,000 police officers will be on duty for the Thames pageant.