The Queen has joined held in her honour.
Waving to the 10,000 ticket-holders on The Mall, the monarch, dressed in a bright pink coat and hat, proceeded from Buckingham Palace down the London thoroughfare in the state review vehicle.
With the Duke of Edinburgh at her side, she smiled at the party-goers from the open top "Queen mobile" as they cheered.
The weather held off as the sovereign arrived. The sides of The Mall were transformed into an open-air picnic where guests celebrated the Queen's official 90th birthday by watching a carnival-style parade, from hundreds of tressel tables.
But persistent heavy showers have fallen for much of the day.
Guests - many from the charities the Queen supports as patron - pulled on white sheer Patron's Lunch ponchos to sit down and eat their picnic hampers filled with Pimms, pork pies and cupcakes.
The Queen was earlier spotted peering out of a window on the palace balcony to see how proceedings were progressing in the damp conditions.
The Duke and Duchess of Cambridge, Prince Harry and other royals met revellers during a walkabout.
Peter Phillips, the Queen's grandson, masterminded the party to mark the monarch's patronage of more than 600 charities and organisations.
It brings to a close this weekend's events marking the Queen's milestone anniversary that has already seen a thanksgiving service at St Paul's Cathedral and a traditional Trooping the Colour ceremony.
Tickets for the event were priced at £150 with the majority going to organisations or charities that have the Queen as patron and 2,500 sold to the general public.
Mr Phillips is a director of Sports and Entertainment Ltd, a global events agency which came up with the idea for the not-for-profit event and will receive an undisclosed fee for staging it.
Pamela Mayne and her husband Stephen, from County Durham, were among the thousands who shrugged off the weather.
Mrs Mayne, 60, dressed in a poncho, said: "We couldn't get any wetter."
She added: "It feels like I'm in a tent in this poncho. It's so British. We're going to enjoy ourselves regardless."
Mr Mayne, also 60, who bought his tickets in the public ballot, said of whether it was worth £150: "In the rain, I don't know. Yes if it had been sunny."
Party-goers were banned from bringing umbrellas on to the site, with organisers handing out ponchos - 12,500 of which they had ready and waiting.
Sitting at a rain-soaked table was Anne Nutt, a member of the Essex Scout executive, who was at the event as Harlow District Scouts had won the Queen's Award for Voluntary Services last year.
Looking around at the large puddles forming in The Mall, Ms Nutt said: "I think today is absolutely marvellous and it's when the British people show their true colours - the true British spirit.
"People have come out from their warm, dry homes for four or five hours in the pouring rain.
"But it's been nice to meet people from other charities and the large number of ex-servicemen that are here."
Ms Nutt said she was making sure her lunch hamper was out of the rain: "I'm keeping it sealed to the last possible moment. We can all drink tea but soggy sandwiches is pushing it."
The Mall was a sea of ponchos, included in the hampers which the guests had received, and there were long queues for tents serving free tea.
One guest, who did not want to give her name, exclaimed: "I've paid £150 for this" as she looked around at the sodden conditions.
Audrey Asquith, from Reading, sat at a table in her poncho eating an ice cream.
She said: "I think this is wonderful, I would have walked through fire (to be here)."
Mrs Asquith and her husband had bought their tickets for the Patron's Lunch and they had spent Saturday night in London to ensure they would not be late for the start of the event.
She added: "We can't be bothering about the weather, otherwise we wouldn't go anywhere in England."
TV and radio presenter Clare Balding appeared to gee up the crowds and, speaking on the stage where William and Harry are expected to pay tribute to their grandmother, told guests "you are rocking those ponchos".