By Bryony Jones and Laura Smith-Spark, CNN
LONDON (CNN) - The Thames became a sea of red, white and blue Sunday, as tens of thousands gathered to celebrate the diamond jubilee of Queen Elizabeth II -- so perhaps it was only fitting that alongside all the Union Jacks, another great British tradition was very much in evidence: Gray skies and heavy rain.
Some 20,000 people are taking to the water aboard 1,000 vessels for a river pageant featuring dragon boats, a floating belfry and the royal barge. The event -- inspired by regal riverside celebrations of the past -- looks set to be the largest such celebration on the Thames for hundreds of years.
And despite the very British weather, up to a million people are still expected to line the route to cheer on the queen, at the head of a seven-mile long flotilla.
Isabella Hales and her family staked out their claim to a spot near Tower Bridge -- where the festivities will climax on Sunday evening -- at 8.30 a.m.
"It's cold, but I don't mind," the 10-year-old, wearing a cardboard Duchess of Cambridge mask that was rapidly dissolving in the drizzle, told CNN. "It was raining for the queen's coronation too. I'm just really excited, I can't wait."
"It's only the second time someone has reigned for 60 years," her aunt Laura Hales added. "It's a big accomplishment, and we wanted to celebrate that.
"There are about 20 of us -- we've come well prepared," she said, pointing out picnic supplies, party masks of the royal family -- including a corgi -- and pink champagne, "And we don't care what the weatherman says.
"Here's to Liz!" she toasted, raising her glass.
Margaretta Soulsby, from Dorset, was the first to arrive at Tower Bridge on Saturday. She had planned to camp out, but when it began raining, stewards persuaded her to spend the night in a tent nearby.
Soulsby told CNN it was "well worth it -- I'm in the perfect position," and said such events made her very proud to be British.
"In 1935, when I was 10, my father took the family to The Mall to watch the silver jubilee celebrations for King George V and Queen Mary, and I've been privileged to be present at all of the major royal events since then."
Roy Clayton traveled from West Yorkshire to be in London for the celebrations. "We're not going to see another monarch reach a milestone like this in our lifetime, so we felt it was important to be here."
Clayton and his wife Elaine were trying to keep out of the cold and rain, huddled under an enormous Union Flag.
"It's a proper one, for a flagpole, and we normally hang it out of the window at home on big occasions, but today it's keeping us warm."
After gathering upriver in west London, the flotilla will wend its way from Battersea Bridge to Tower Bridge, passing through the heart of Britain's capital city over the course of several hours.
At the front will be 300 man-powered boats, with thousands of volunteers propelling them downriver, flags and streamers fluttering around them.
A barge carrying the eight Royal Jubilee Bells -- the largest of which, at nearly half a ton, is named for the monarch -- will lead the way, its peals of bells to be echoed from church towers along the river.
Next will come passenger boats, pleasure boats, historic wooden vessels -- the oldest built in 1740 -- and boats carrying members of the armed forces, police and fire services. One of the boats taking part, the Amazon, also took part in the 1897 Diamond Jubilee celebrations for Queen Victoria, Britain's longest-serving monarch and the only other to reach the landmark 60 years on the throne.
The biggest cheer will no doubt come for the present queen, who will be carried aboard a specially converted royal barge, opulently draped in red and gold.
Sailing boats that are too tall to pass under the 14 bridges along the river pageant route will line the river from London Bridge to Wapping, in the east, creating an avenue of sails set against the Tower of London and the city's financial center.
The queen will disembark at Tower Bridge and watch as the remainder of the river pageant passes in a riot of color and noise.
More than 14 miles of bunting in the colors of the Union Flag, or the Union Jack as it is commonly referred to, will be strung along the route, pageant organizers say.
London's Metropolitan Police said as many as 6,000 extra officers would be on patrol during jubilee events. The force will deploy more than 20 police boats during the river pageant, as well as carrying out searches on shore with dogs, and using rope climbing teams to inspect the bridges. Police divers will carry out underwater checks of piers and boats.
The huge security operation comes as London prepares to host the 2012 Olympic Games, which open in late July.
Outside the capital, Britons are expected to gather for thousands of jubilee-themed street parties and barbecues Sunday. Stores have been filled for weeks with an array of patriotic paraphernalia, from flag-adorned teapots to aprons to picnic sets, to help hosts set the scene for what is billed as a national celebration.
The celebrations continue on Monday and Tuesday, which have been declared public holidays to mark the diamond jubilee.
An afternoon garden party at Buckingham Palace will be followed Monday evening by a televised pop concert outside the palace grounds.
At the end of the concert, the queen will take to the stage to light the "National Beacon," which will be on the Mall. She will use a diamond made from crystal glass, which has been on display at the Tower of London from the beginning of May, to light the flame.
More than 4,000 beacons will then be lit in communities throughout the United Kingdom, along with the Commonwealth and UK Overseas Territories.
Tuesday will be a day of pomp and ceremony, as the queen attends a service of thanksgiving at St. Paul's Cathedral, followed after lunch in Westminster by a carriage procession back to Buckingham Palace, where she will appear on the balcony, flanked by members of the royal family.