LONDON — Under the rainy skies of London, the crowds acclaim for  the monarch marking 60 years on the throne, a flotilla of 1,000 boats set off down the Thames on Sunday in an extravagant royal pageant. Thousands of Londoners are lining the Thames in London for the Jubilee river pageant despite the dank weather, as street parties get under way across the nation.

The spectacle, along a seven-mile stretch, will end six hours later when the last boat passes Tower Bridge.

The Thames barrier is being lowered to slow the river’s flow. Some 20,000 people are expected to be in the boats of the flotilla, which will travel at 4 knots (4.6 miles) an hour.

Despite cool, drizzly weather, hundreds of thousands of people are expected to line the riverbanks between Hammersmith and Tower Bridge in London, feting the British monarch whose longevity has given her the status of the nation’s favorite grandmother.

The flotilla, the largest such pageant for 350 years, will range from rowing boats to military boats, including a barge carrying the Queen and other members of her family. The queen and members of her family will lead the river pageant aboard a flower-bedecked royal barge, accompanied by skiffs, barges, narrowboats, motor launches, row boats and sailing vessels from around the world.

Other vessels will include a contingent of Dunkirk “little ships” and tall ships such the Southampton-based Tenacious, which will be sailed by a mixed crew of able-bodied and disabled people.

More than 20 vessels from the Royal National Lifeboat Institution (RNLI) are being drafted in for the occasion to help with safety on the river. Another vessel will take on a ceremonial role during the pageant in celebration of the 60,000 lives saved by the RNLI during the Queen’s reign.

The 86-year-old queen, accompanied by her 90-year-old consort Prince Philip and an array of family members, was scheduled to spend two hours moving slowly down the river aboard a lavishly adapted royal barge, the Spirit of Chartwell, that was emblazoned for the occasion with a coat of 10,000 cut flowers.

Only once before in the 1,000-year history of the British monarchy had any reigning sovereign lived long enough to mark a diamond jubilee, and that was in 1897, when Queen Elizabeth’s great-great grandmother, Queen Victoria, then aged 78, achieved the landmark.

“We in Britain are experts at not letting the weather spoil our fun,” said Adrian Evans, pageant master for Sunday’s flotilla. “The London Philharmonic Orchestra will be playing ‘Singin’ In The Rain’ as they travel down the river, and the crowd can sing along with them.”

Hundreds of people ignored the persistent rain and camped out overnight to secure prime riverside spots. Crowds swelled into the thousands Sunday, with revelers in hats, flags, leggings and rain ponchos adorned with the Union flag mixing with burger and cotton candy vendors along the 7-mile (11-kilometer) route.

“It would have been wonderful if it had been sunny like last Sunday but we have come prepared,” said 57-year-old Christine Steele. “We have got blankets, brollies (umbrellas), flags and bunting. We even got our glittery Union Jack hats and wigs, and the Champagne is on ice.”

Not everyone in Britain is celebrating. The anti-monarchist group Republic held a riverbank protest Sunday to oppose the wave of jubilee-mania.

“People are sick and tired of being told they must celebrate 60 years of one very privileged, very remote and very uninspiring head of state,” said the group’s chief executive, Graham Smith. “The hereditary system is offensive to all the democratic values this country has fought for in the past.”

Jubilee celebrations kicked off Saturday with a royal day at the races, as the queen watched a horse with the courtly name of Camelot win the Epsom Derby.