Most British papers carried a front-page picture of Prime Minister Theresa May signing the formal notice.
Most British papers carried a front-page picture of Prime Minister Theresa May signing the formal notice of Britain's intention to leave the bloc, triggering two years of divorce negotiations.
The Sun, Britain's most popular newspaper, triumphantly beamed "SEE EU LATER" and "DOVER AND OUT" onto the White Cliffs of Dover, the closest point to continental Europe.
"It's finally here. The most momentous day in Britain's modern history," the tabloid said.
The paper said it had enormous optimism about a Britain better able to control its own laws, borders and trade deals.
It blasted the "sneering, divisive rage of hardcore Remainers... their apocalyptic rhetoric` at a comical crescendo", who will be left "howling at the moon" after Wednesday.
"Today it begins for real -- and we can't wait."
"FREEDOM!" cried the Daily Mail, which also had sharp words for Brexit doubters.
"This is a momentous day for Britain -- a day many who have grown disillusioned with the EU feared they would never see," it said.
The Daily Telegraph broadsheet had a cartoon of May as Buzz Lightyear from "Toy Story" standing on a Union Jack bedsheet shouting "To infinity and beyond!!"
"For those who have for so long dreamed of this moment, a toast is irresistible," it said.
"To the triggering of Article 50 and the accomplishment of an impossible dream. And to Britain's whose future is full of potential."
The Daily Express's editorial said: "This is a joyous day indeed."
"Today we formally begin the process of reclaiming the right to decide our own laws, how our taxes are spent, whom we trade with and who gets to come into this country."
Newspapers which had backed Britain staying in the European Union sounded notes of caution on the two years of departure talks ahead.
"The eyes of history are watching", The Times said on its front page along with a picture of May putting ink to paper under a portrait of Britain's first prime minister, Robert Walpole.
The Guardian's front page was a jigsaw map of Europe with Britain taken out.
"Today Britain steps into the unknown," its headline said.
Its editorial said: "It is in the interests of both sides in the negotiations to work to reconstruct a new relationship that does least harm.
"The challenge is that Theresa May's objectives all tend towards a result that will unquestionably disrupt the British economy and likely inflict collateral damage on the rest of the EU."
The Financial Times said a softer tone from London on the role of the European Court of Justice and the Brexit divorce bill in the run-up to negotiations "opens way for compromise".
"Dear EU, it's time to go," said the Daily Mirror's front page.
"Historians will look back and study this turbulent period in our nation's story. We want them to find a united, relieved country," the editorial said.
Europe's major newspapers were mostly sombre and questioned whether all would turn out well.
In Germany, the Berliner Zeitung addressed "a letter to London", insisting, "it's not too late".
Die Welt used German-accented English to declare: "Dear Brits, ze door is schtill open", while Frankfurter Allgemeine Zeitung said breaking up was hard to do.
"Now it's getting serious," it said. "No-one can say with certainty what will happen over the next two years. One thing's for sure: it'll be no picnic!"
Italy's La Stampa focused on the fears of UK-based Italians over the uncertainty surrounding their future.
In France, Le Monde said "Madame Brexit unleashes the divorce," while Le Figaro had a double-page spread on the issues at stake in the "titanic project" of Britain's "adieu" to Europe.
"Miss you already! Or not," said Liberation, imagining two scenarios.
In one, Brexit is successful and Britain stays united, with strong economic growth.
In the other an "economically strangled" Britain, having lost Scotland and Northern Ireland, "comes back, tail between its legs, demanding to rejoin a buoyant EU".