The Thames became a sea of red, white and blue Sunday, as tens of thousands celebrated the diamond jubilee of Queen Elizabeth II -- so perhaps it was only fitting that alongside all the flags, another great British tradition was very much in evidence: Gray skies and 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 were still expected to line the route to cheer on the queen, at the head of a seven-mile long flotilla.

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Isabella Hales and her family staked out their claim to a spot near Tower Bridge -- where the festivities reached a climax on Sunday evening.

"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 made 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 were 300 man-powered boats, with thousands of volunteers propelling them down river, 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 -- led the way, with peals of bells ringing out from church towers along the river.

Next came 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 cheers were reserved for the present queen, who was carried aboard a specially-converted royal barge, opulently draped in red and gold.

Sailing boats that were too tall to pass under the 14 bridges along the river pageant route lined 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 disembarked at Tower Bridge and looked on as the remainder of the river pageant passed by in a riot of color and noise. Excitement grew as a gun salute rang out from the Tower of London.

Nearby, those not lucky enough to get a riverside spot before the area was locked down, watched the pageant on a big screen. Cheers, whistles and the odd chorus of "God Save the Queen" rang out, and the crowd stayed jolly despite the rain.

Helen McKee, from Kent, said she bought her family along to enjoy a once-in-a-lifetime spectacle. "We're never going to see this sort of thing again. I've got a little boy and I thought it was important for him to see it. I still remember the Silver Jubilee in 1977. It's a great atmosphere here, everyone is so friendly."

To Jamie Newell, from London, events on the river were just a forerunner of the main attraction of the day - the after party he was planning at home. Newell, decked out from head to toe in Union flags, and sporting red, white and blue contact lenses, said simply: "I love the queen."