St Andrews, the Fife town where Prince William and Kate Middleton met, hosted a breakfast for 1,500 people.
And David Cameron is holding a party in Downing Street, where cupcakes baked by his wife Samantha will be served.
The scenes have been replicated in streets, squares, pubs and churches across the UK, including the bride's home village of Bucklebury, Berkshire.Fancy dress
A wedding breakfast was held in the hamlet of Chapel Row where the Middletons live, and Bucklebury Farm Park is staging a day of celebrations culminating in a proms-style fanfare.
The wedding was broadcast live on big screens in cities and towns, including in Southampton where people were encouraged to dress in patriotic red, white and blue.
Several thousand people turned out, including Michaela Coutakis, 45, who said: "We came today because it's an important national occasion.
"We're not royalists but it's bringing the country together and it's quite exciting really. We will remember this when we're old and grey and we can say we came down to the square to watch Kate and William get married."
Hundreds of revellers held a fancy dress party on the restored £39m Grand Pier in Weston-super-Mare, Somerset.Pier owner Kerry Michael, said: "It is buzzing here. The atmosphere is electric. The first people got here at around 6am for a champagne breakfast. By nine there were hundreds here."
In Cornwall, which has strong ties with the Prince of Wales, there were 30 applications for road closures so festivities could be held.
Gloucester Cathedral is showing the wedding on a giant screen, and parties are planned in the city and nearby Cheltenham.
A mass picnic is happening along the High Street of Alcester, in Warwickshire.
In Tetbury - the closest town to Highgrove House, the family home of the Prince of Wales and the Duchess of Cornwall - people are also enjoying picnics
In Eastbourne, East Sussex, hundreds turned up around a huge screen in Princes Park.
There were 16 street parties held in Edinburgh, including one in Broughton Street for 250 guests.
In Leeds, where Kate Middleton's father Michael was born, celebrations are taking place across the city, and York is staging a fancy dress competition and high tea.
A number of street parties are going on in Sheffield, with the focus in the city centre at Devonshire Green.
People in Hull are also joining the celebrations, with six street-party applications approved in the city, contrary to reports a few weeks ago saying no-one had applied.A breakfast was held in St Andrews where Prince William met Kate Middleton
The Marple Bridge Association in Cheshire, meanwhile, has been planning its town centre party since Christmas, and has sold 800 tickets.
Almost 250 street parties are being held across Wales, with Cardiff leading the way with more than 50.
On Anglesey, where the couple will live, about 2,000 people watched the Bunting out for wedding parties ceremony on a giant TV screen at the island's agricultural showground.
Whilst in Londonderry, fans have dressed in full wedding attire for royal breakfasts in community centres. Events have also taken place in Belfast, Newtownards, and Templepatrick.
In London, big screens have also been erected in Hyde Park where a crowd of thousands cheered as William and Kate exchanged their vows.
Several hundred people gathered in Manchester city centre as the wedding was shown on a big screen.
Elaine Lowe, 22, who came with a group of friends, said: "The rain has held off, the sun is out, and we are all having a great day.
"Kate looks stunning and her wedding dress is fabulous. What's not to like about today?"
In Scotland, visitors at royal residence Balmoral Castle watched the ceremony on six big screens.The event in Hyde Park in central London attracted several thousand
Hundreds brought picnics and drank champagne as they sat on the lawn outside. The royal couple have enjoyed several breaks at Prince of Wales' home on the estate.
Meanwhile, the breakfast event in St Andrews was televised worldwide.
Elsewhere in the town, John Montgomery, 44, from the Tayport area, said: "St Andrews feels responsible for all of this.
"The atmosphere's fantastic here. It's amazing really for a small Scottish town to have two billion people around the world watching it."
But in Edinburgh, republican demonstrations were in the Royal Mile, and a large unofficial party in Glasgow has been criticised by the local council.
Two parties have also planned in Bristol for those who want to avoid wedding fever; Hebden Bridge in Yorkshire is staging a "Not The Royal Wedding Party" event, and campaign group Republic hosted a "Not the Royal Wedding" street party in central London.
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