A pro-Brexit supporter pours beer onto an EU flag in Parliament Square, London, ahead of the UK leaving the European Union at 11pm on Friday.

Hundreds of people celebrating Brexit congregated at Parliament Square on Friday night.

The gathering was generally good-natured - though it was punctuated by a notable act of protest when one man poured the contents of a can of Stella Artois (the “reassuringly expensive” Belgian beer) over an EU flag.

The unidentified Brexiteer, wearing a navy hoodie and jeans, was cheered on by other pro-Brexit revellers, some of whom filmed it on their phones.

The flag appeared to have already been trampled and mashed into the mucky grass by revellers at Nigel Farage’s Leave means Leave party.

View photos A pro-Brexit supporter pours beer onto an EU flag in Parliament Square, London, ahead of the UK leaving the European Union at 11pm on Friday. More

Hundreds of people turned up to hear speeches from some of the main Brexit players, including the Brexit Party leader and Wetherspoons chairman Tim Martin.

In unison, the crowds counted down together and at 11pm, the UK officially left the European Union after 47 years of membership.

The shift has been more than three years in the making since the 2016 referendum with a seemingly constant back and forth between the two main parties.

View photos Pro-EU campaigners take part in a 'Missing EU Already' rally outside the Scottish Parliament, Edinburgh. (PA) More

The historic moment was marked by both celebrations and anti-Brexit protests.

A Missing EU Already rally took place outside the Scottish Parliament. It was organised by the Edinburgh Yes Hub, which backs Scottish independence.

In Northern Ireland, the campaign group Border Communities Against Brexit is to stage a series of protests in Armagh, near to the border with the Republic of Ireland.

View photos Nigel Farage speaks to pro-Brexit supporters in Parliament Square, London. (PA) More

View photos Pro-Brexit supporters celebrate in Parliament Square shortly after Britain officially left the EU. (PA) More

In London, Pro-Brexit supporters sung “bye, bye EU” to the tune of Auld Lang Syne

In a series of highly symbolic events earlier, the UK's flag was removed at the headquarters of the European Council in Brussels.

And the EU flag was taken down at the British government's building in the Belgian capital.