There may be "bumps in the road" but Brexit can "unleash the full potential" of the UK, Boris Johnson has said.
As the UK's departure from the European Union was sealed last night, the Prime Minister acknowledged mixed feelings in a still-divided nation about the end of 47 years of close links with Brussels.
In Northern Ireland, pro-EU activists demonstrated on the Irish border as Brexiteers held a party at the gates of Stormont to celebrate the landmark moment.
First Minister Arlene Foster, appearing on The Late Late Show in Dublin, said she "understood very much" the anxiety of people who wanted to remain in the EU and the many people who had anxieties.
The UK joined the then European Economic Community in 1973 but the 2016 referendum signalled the beginning of the process which resulted in Britain's membership of the bloc coming to an end at 11pm last night.
Addressing the nation last night, Mr Johnson said: "For many people this is an astonishing moment of hope, a moment they thought would never come.
"And there are many of course who feel a sense of anxiety and loss."
Mr Johnson said his job was now to "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," he said.
"And whatever the bumps in the road ahead I know that we will succeed."
On a day filled with symbolic moments:
π The Union flag was removed from the European Union institutions in Brussels
π Brexiteers gathered for a party in Parliament Square, with some jumping on an EU flag in the mud
π The White Cliffs of Dover were used to send pro-EU messages
π A patriotic red, white and blue light show illuminated official buildings in Whitehall and Union flags lined The Mall.
In Northern Ireland, a crowd of pro-Brexit supporters gathered at the gates of Stormont to count down to 11pm when the UK officially left the EU.
Many were holding Union flags and toasted the moment Brexit became official. A Lambeg drum was played and Chinese lanterns were released at 11pm.
Appearing on The Late Late Show, Mrs Foster said: "We are leaving the European Union tonight, the United Kingdom leaves, but that doesn't mean we are not still neighbours and I wanted to send out a message that we are, and will continue to be, neighbours."
In a wide-ranging interview, Ms Foster insisted that Brexit would not bring a united Ireland any closer.
Earlier, DUP MP Sammy Wilson, a vocal Brexiteer, said the UK was breaking free from the "EU prison" but warned that Northern Ireland could not be left behind.
Sinn Fein president Mary Lou McDonald said the refusal of Mr Johnson to contemplate extending the transition period beyond the end of 2020 was "cause for concern".
As the minutes ticked down to the 11pm departure, Nigel Farage took to the stage at the Brexit celebration rally in central London.
Speaking to the crowd at Parliament Square, the Brexit Party leader said: "We did it. We transformed the landscape of our country.
"There are some that say we shouldn't celebrate tonight, but we are going to celebrate tonight.
"There is one thing above all we must celebrate tonight and it is this: The reason we are here tonight is because Westminster became detached from the people in this country.
"The people have beaten the establishment. The real winner tonight is democracy. And I am someone who believes we should be pro-Europe, but not the European Union."
UK citizens will notice few immediate changes as the country officially leaves the EU.
Most EU laws will continue to be in force - including the free movement of people - until December 31, when the transition period ends.
The UK is aiming to sign a permanent free trade agreement with the EU - similar to the one the EU has with Canada - by then.
Mr Johnson has been clear he also wants to strike deals with countries around the world, notably Donald Trump's USA.
But Brussels is pessimistic about the 11-month timetable for reaching a deal and made clear that Britain will have to accept worse terms and conditions for trade than if it were still a member of the EU.