LONDON — Well, that wasn’t easy. But after 1,317 days, three prime ministers, two blown exit dates, dozens of votes in Parliament and years of negotiation, Britain finally left the European Union on Friday.

As of 11 p.m. London time, midnight in Brussels, Britain was out of the bloc it helped establish almost five decades ago.

Brexit will barely be felt by average citizens until the end of an 11-month transition period. But Britain’s departure will profoundly change its relationship with Europe and the rest of the world.

The United Kingdom will go it alone — and either zoom ahead with Prime Minister Boris Johnson’s vision for a can-do “Global Britain” or find itself economically diminished, a solo player on the world stage, spending years seeking trade deals and relevance, with its bowler hat in hand.

As the big moment approached, a countdown clock was projected onto the facade of Downing Street.

Around the corner, at a party in Parliament Square, hard-core Brexiteers were rowdy and merry in the drizzle. Attendees wore Union Jacks as capes. Blokes drank cans of beer — most brewed in Europe, like San Miguel and Stella Artois. The crowd skewed older, with plenty of bald and gray heads.

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“Go, Boris!” some shouted.

“I say we’re very satisfied, very satisfied,” said Anne Scott, 52, from the Midlands, who waved a small flag she’d bought for 2 pounds outside Westminster tube station. “Finally we are free, that’s what it feels like, to be honest.”

Nigel Farage, leader of the Brexit Party and host of the gathering, told supporters: “We did it. We transformed the landscape of our country.” The throng sang Queen’s “We Are the Champions” and the British national anthem, “God Save the Queen.”

The prime minister didn’t want to gloat, with pollsters finding that more than half the country opposes Brexit. But he did want to mark the moment.

In a prerecorded speech broadcast an hour before the official departure, Johnson attempted to strike an upbeat note and also promised to heal a divided country.

“This is the moment when the dawn breaks and the curtain goes up on a new act in our great national drama,” Johnson said.

Britain’s departure is a victory for anti-establishment forces and a sharp rebuke to the European project’s fundamental belief that peace and prosperity are best achieved by opening borders and deepening integration, and that coalitions make individual countries stronger rather than taking away their sovereignty.

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It brought the U.K. into closer alignment with the hammer-swinging mood of President Donald Trump, who has embraced the breakup.

Trump “has long supported the United Kingdom’s sovereign decision to withdraw from the European Union,” U.S. Ambassador Woody Johnson said in a statement Friday, adding that he looked forward to “even stronger” U.S.-U.K. relations in the Brexit era.

In his address, the populist Johnson declared that “for all its strengths and for all its admirable qualities, the EU has evolved over 50 years in a direction that no longer suits this country.”

At 10 Downing Street, Johnson hosted an evening party for ministers and advisers. The Guardian revealed the menu: “English sparkling wine, shortbread topped with Shropshire blue cheese, filet of lamb, a ploughman’s of cheddar and pickle, roast chicken skewers — probably not chlorinated — and roast beef and Yorkshire pudding.”

It was nationalist, no doubt, but the vibe was low key.

Steve Baker, a leading Brexiteer in the British Parliament, conveyed the dominant mood: “I will celebrate. I will allow myself a smile. I’ll allow myself that glass of champagne. I will enjoy myself. But I will celebrate discreetly, and I will celebrate in a way which is respectful of the genuine sorrow that others are feeling at the same time.”

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In Brussels — the home to Brits who have devoted their careers to the EU and to the Europeans who have lived and worked alongside them for decades — there were few celebrations, with all but a handful of Brexit Party lawmakers mourning the departure.

At a time when the institutions that have preserved stability and advanced democracy in the generations since World War II are under attack on both sides of the Atlantic, Brexit is a reversion, at least for one nation, to the go-it-alone attitude of a much earlier era.

Britain’s departure is also a loss for the EU, whose free-trading, expansionist vision of the world was profoundly steered by expertise and ideas from British leadership, no matter how much the country’s tabloids ginned up stories of humiliation at the hands of dastardly continentals.

Now Britain will stand apart, as, in the words of Irish leader Leo Varadkar, “an old friend embarking on an adventure.”

Once night fell on Friday, U.K. flags started to come down from EU buildings, silently and unceremoniously, the tectonic political split reduced to a few brisk gestures from maintenance personnel. At the U.K. mission, a man in a suit opened a window, yanked down the EU flag from underneath the Union Jack, and slammed it shut again.

Janitors have been instructed to take away the British seat at EU meeting tables over the weekend, now that only 27 chairs will be needed. EU security staffers scrubbed their databases Friday so that British diplomats can no longer access internal EU documents or use their identification cards to enter EU buildings. On Thursday, an email was sent to EU staffers reminding them that they should no longer share documents with British officials.

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Inside the European Parliament, British lawmakers’ names were stripped from a bank of mailboxes. “Dear Members,” read an improvised sign, “Due to Brexit and the subsequent new distribution of seats, we are obliged to rearrange the post boxes.” The note added: We “will do everything we can to keep this inconvenience as short as possible.”

During the transition period, Britain must continue to obey EU rules, even though it is no longer helping shape them. British members of the European Parliament have lost their jobs. The British prime minister will no longer sit on the European Council.

Through the end of 2020, negotiators in Brussels and London will seek to hammer out a new trade deal and come to terms with pacts for data-sharing, security, fisheries, aviation, banking and much, much more. Or at least that is the plan.

A joint letter published on Friday by EU leaders — European Commission President Ursula von der Leyen, European Council President Charles Michel and European Parliament President David Sassoli — signaled that they will fight for their interests in upcoming talks.

“Without the free movement of people, there can be no free movement of capital, goods and services,” they wrote. “Without a level playing field on environment, labour, taxation and state aid, there cannot be the highest quality access to the single market. Without being a member, you cannot retain the benefits of membership.”

EU citizens will be able to continue to live and work in Britain, and Brits in Europe, through the transition period, but after that they will need to apply for residency rights.

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In an address Friday evening from Paris, French President Emmanuel Macron suggested Brexit should serve as a “historic alarm signal.”

“The Brexit is possible, was possible, because we have too often made Europe a scapegoat for our difficulties,” Macron said.

The fact that the United Kingdom is still deeply split over the issue of Brexit was reflected not just in the polls but also in Friday’s commemorations.

In addition to the celebrations, there were anti-Brexit events throughout the country: A candlelight vigil outside the European Commission building in London, a “European Solidarity March” in the historic town of Canterbury, protests in villages along the border between Northern Ireland and the Republic of Ireland.

In a sign of trouble brewing north of Hadrian’s Wall, Scotland First Minister Nicola Sturgeon used a speech in Edinburgh to renew the press for a second referendum on Scottish independence.

Led by Donkeys, an anti-Brexit activist group, on Friday projected onto the White Cliffs of Dover a video featuring two World War II veterans. It ended with an image of the EU flag, each of its stars fading away until there was just one, representing Britain’s membership. “This is our star,” the caption read. “Look after it for us.”

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The Brussels public transportation system produced a video with a wistful soundtrack that highlighted all the tram stops named after British people and places — Liverpool, Churchill, Montgomery — many commemorating Britain’s role in liberating Belgium from Nazi occupation.

The video ended with a play on the London tube’s famous warning: “Mind the gap when leaving.”

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Birnbaum reported from Brussels. The Washington Post’s Quentin Ariès in Brussels and James McAuley in Paris contributed to this report.