There may be "bumps in the road" but Brexit can "unleash the full potential" of the UK, Boris Johnson has said.
As the moment of the UK's departure from the European Union approached, the British Prime Minister acknowledged mixed feelings in a still-divided nation about the end of 47 years of close links with Brussels.
"For many people this is an astonishing moment of hope, a moment they thought would never come," he said.
"And there are many of course who feel a sense of anxiety and loss."
After years of bitter wrangling since the 2016 referendum, 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.
"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."
Tonight we are leaving the European Union. pic.twitter.com/zZBsrf4BLe— Boris Johnson (@BorisJohnson) January 31, 2020
Across the course of today the Union flag was removed from the European Union offices in Brussels, while a party at Parliament Square in London saw some jumping on an EU flag in the mud.
Meanwhile the White Cliffs of Dover were used to send pro-EU messages and a 50p coin to mark the occasion entered circulation.
With little set to change at 11pm as a result of the deal negotiated by the UK and EU, attention has already turned to the next set of talks aimed at securing the future relationship which will apply from January 1, 2021.
And Mr Johnson has been clear he also wants to strike deals with countries around the world - notably Donald Trump's USA.
A Downing Street spokesman said: "Cabinet discussed the Government's future trade agenda, which includes seeking a Canada-style FTA with the EU.
"From tomorrow, the UK will also be free to begin trade negotiations with countries around the world - with the aim to have 80% of our trade covered by FTAs within three years."
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.
European Commission president Ursula von der Leyen said: "We want to have the best possible relationship with the United Kingdom, but it will never be as good as membership."
Senior Cabinet minister Michael Gove acknowledged the trade-offs that would have to be made in any deal.
"We want trade to be as frictionless as possible but the EU is clear, you can only have fully frictionless trade if you accept all of their rules, if you accept all their laws, you are subordinate to their judges, you are subordinate to their political structures," he told the BBC.
There "will be some regulations that will differ in Britain" so "that may mean that when it comes to trading with Europe there are some bureaucratic processes there that aren't there now".
As the UK Cabinet gathered in Sunderland, car giant Nissan - which has a major plant in the city - called for clarity from the Government about its plans and urged both the UK and EU to secure "an orderly balanced Brexit that will continue to encourage mutually beneficial trade".
Mr Johnson, who has promised a "dignified" and "respectful" approach to the Brexit celebrations, was spending the evening in Number 10 at a reception for senior ministers, officials, and supporters of the Vote Leave campaign to leave the EU.
Guests were quaffing English sparkling wine and dining on canapes including fillet of lamb on toast, Shropshire blue cheese, beef and Yorkshire pudding with horseradish sauce, mushrooms tarts and roast chicken skewers.
In Scotland, which voted to stay in the EU in the 2016 referendum, rallies and candlelit vigils were taking place.
The Leave a Light On gatherings were intended to send a message to the EU to keep open a place for Scotland.
In Northern Ireland, the campaign group Border Communities Against Brexit staged a series of protests in Armagh, near to the border with the Republic of Ireland.