- Video report by ITV News Political Correspondent Paul Brand
A new era for Britain has started after the country left the EU, more than three years after Britain voted for Brexit.
After 47 years of membership, the UK left the EU at 11pm on Friday, with Downing Street marking the occasion with a light show.
It has taken three prime ministers and three Brexit secretaries for the UK to reach this stage - and the process is still not yet complete.
An 11-month transition period begins where the UK will look to negotiate a trade deal with the EU.
In a message to Britons, the European Parliament's Brexit co-ordinator Guy Verhofstadt said he would "work to ensure the EU is a project you'll want to be a part of again soon".
On Friday night, hundreds packed out Westminster's Parliament Square for a Brexit celebration party but in places such as Scotland and Brussels it was a moment of sorrow.
Those outside Parliament were joined by Brexit Party leader Nigel Farage, who joined crowds in singing the British national anthem as the clock struck 11pm.
Police said protests and celebrations went to plan, with just a handful of arrests across the whole of the United Kingdom.
Mr Farage, who has worked toward Brexit for most of his political career, described it as "the greatest moment" in British history.
"Let us celebrate tonight as we have never done before.
"This is the greatest moment in the modern history of our great nation."
And on social media dozens of users reported hearing fireworks as revellers celebrated Brexit.
ITV News Scotland Correspondent Peter Smith described a gathering outside the Parliament in Holyrood as a "vigil to mourn" the nation's departure from the EU.
Hundreds of disappointed Scots held European flags and sang Auld Lang Syne as the UK left the bloc.
At 11pm, Scotland First Minister Nicola Sturgeon tweeted a picture of the EU flag, adding: "Scotland will return to the heart of Europe as an independent country - #LeaveALightOnForScotland".
Outside the European Parliament, Europe Editor James Mates said Brexit Day was a "night that the 27 others didn't want to happen".
It was a night, as Mates said, that many European leaders didn't actually believe would ever happen.
When David Cameron called a referendum in 2016, few could have imagined the way politics would grind to a halt in the aftermath.
Theresa May was unable to get her Brexit deal through, leading to her resignation as PM and Boris Johnson taking over.
- Watch: Britain's 47 year relationship with the EU in three minutes
He called a snap election, defeated Labour and, with a huge majority in Parliament, brought the UK out of the EU, bringing an end to more than four decades of partnership.
The prime minister marked the historic moment by urging people to embrace a "new era of friendly cooperation between the EU and an energetic Britain".
While hundreds of Brexit supporters celebrated the moment in Parliament Square, Mr Johnson sought to reassure those "who feel a sense of anxiety and loss" that his priority is to now "bring this country together".
"We want this to be the beginning of a new era of friendly cooperation between the EU and an energetic Britain, a Britain that is simultaneously a great European power and truly global in our range and ambitions.
"And when I look at this country's incredible assets, our scientists, our engineers, our world-leading universities, our armed forces, when I look at the potential of this country waiting to be unleashed I know that we can turn this opportunity into a stunning success.
"And whatever the bumps in the road ahead I know that we will succeed."
In a tweet to mark the moment the prime minister said the UK had reached an "extraordinary turning point".
He admitted there would be "bumps in the road" but said Brexit can "unleash the full potential" of the UK, as prepares for a tense 11 months of further negotiation.
The months that follow this point will, in terms of Brexit, be known as the 'transition period', which will finish, if not before, at the end of 2020.
In that time, Mr Johnson and his team will attempt to negotiate the UK's future relationship with the EU, which will include a Free Trade Agreement.
That work however won't be carried out by the Brexit secretary, whose department wound up at 11pm.
As the Department for Exiting the European Union announced its closure, Stephen Barclay became a backbench MP.
On the United Kingdom's final day in the EU, the prime minster was joined by Cabinet colleagues in Sunderland.
The north east city was the first in the country to return a vote to leave the European Union.
Downing Street said the PM and his team discussed the government's future trade agenda, which includes seeking a free trade agreement with the EU.
As the clock struck 11pm, Britain's MEPs lost their seats in the Strasbourg parliament and have to go back to normal lives.
On Friday afternoon, a pro-EU banner was unveiled on Westminster Bridge by campaigners mourning the UK departing from the trade bloc. Smoke flares were also set off during the protest.