* Four days of festivities begin with Epsom Derby
* Celebrations include Thames pageant and pop concert
* Tributes paid to queen's "incredible service"
By Michael Holden and Mike Collett-White
LONDON, June 2 (Reuters) - Britain's Queen Elizabeth marks 60 years on the throne on Saturday with a visit to the races, indulging a lifelong passion for horses and launching four days of nationwide celebrations to honour a monarch riding high in public affection.
The 86-year-old will watch the Epsom Derby in southern England, joining a crowd of up to 150,000 racegoers dressed in their summer best for one of the racing season's highlights.
The focus then turns to London, where huge crowds are expected to line the streets and the River Thames for a series of spectacular events, although forecasts of rain and unseasonably cold weather could dampen enthusiasm.
Millions more are expected to attend street parties across the country as the nation marks the queen's personal milestone under the banner of the "Diamond Jubilee".
"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 appreciation of a woman who learned she was queen at the age of 25 while on holiday in Kenya with her husband Prince Philip.
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.
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 is the only other monarch to have celebrated a Diamond Jubilee.
As well as 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.
On Sunday, a flotilla of 1,000 boats assembled from around the globe will travel 25 miles along the River Thames to accompany the queen and her 90-year-old husband 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 beacons will also be 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 flypast by present and former royal air force aircraft.
Large crowds are expected, with estimates that about a million people will travel to London on Sunday alone. Not all will be cheering for the queen, however, with banner-waving republicans protesting at Tower Bridge during the 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 also said an estimated 2 million Britons were planning to head overseas to take advantage of the two extra public holidays.
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.