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Jeremy Corbyn after being elected leader of Britain's opposition Labour Party on Saturday.Credit Stefan Wermuth/Reuters

LONDON — After three decades as a political outsider and clarion of the left, Jeremy Corbyn on Saturday won the leadership of Britain’s opposition Labour Party with an emphatic victory and a program that includes expanding the economy, scrapping nuclear missiles and dismantling the centrist policies of his predecessors, including former Prime Minister Tony Blair.

Mr. Corbyn, 66, won the Labour leadership overwhelmingly with the backing of thousands of newly recruited supporters, and in doing so delivered one of the biggest upsets in modern British politics.

His success underlines the extent to which European political structures have been destabilized by the aftershocks of the financial crisis in 2008, with voters increasingly attracted away from the political center ground, either to the socialist left or the nationalist right.

However, Mr. Corbyn’s program, which includes nationalizing energy and rail companies, has shallow support among fellow Labour lawmakers, a fact that suggests he may struggle to unite his party. Several senior party figures, including Emma Reynolds and Tristram Hunt, have already announced that they will not be serving on Mr. Corbyn’s team, though another, Hilary Benn, promised to support him.

On Saturday there were jubilant scenes after the release of results showing that Mr. Corbyn had won almost 60 percent of the vote, crushing his three opponents, Yvette Cooper, Andy Burnham and Liz Kendall.

“We don’t have to be unequal, it doesn’t have to be unfair, poverty isn’t inevitable,” Mr. Corbyn told a cheering audience in a short acceptance speech.

“Things can change and they will change,” he added, denouncing “grotesque” levels of inequality and placing blame for the migration crisis currently sweeping Europe on the bitter legacy of going to war. One of his first acts, Mr. Corbyn said, will be to attend a demonstration in London to highlight the plight of refugees.

Mr. Corbyn’s perceived integrity and his willingness to speak his mind have struck a chord in a party in which many supporters were left disillusioned by the leadership of Mr. Blair, whose decision to join President George W. Bush in invading Iraq poisoned his legacy. Yet Mr. Blair is one of only a handful of Labour leaders who has ever won a general election, and Mr. Corbyn’s critics fear he will turn the Labour Party into a protest movement, rather than a realistic alternative to the Conservative government of Prime Minister David Cameron.

“I’m learning how to grit my teeth and bite my tongue,” one Labour lawmaker, who asked not to be named to avoid alienating Mr. Corbyn, said outside the conference center where the results were made public.

How long Mr. Corbyn’s critics stay quiet remains to be seen. His parliamentary record as a serial rebel who frequently defied the party line will also make it hard for him to appeal for loyalty.

Yet the scale of his win may give Mr. Corbyn some time to establish his leadership. In fact Mr. Corbyn, a vegetarian and teetotaler, had to be persuaded to run as the left’s leadership candidate, and he nearly failed to make the ballot paper.

To stand, Mr. Corbyn needed 35 nominations from Labour lawmakers, and he scraped enough together only when several of those who did not support him decided he ought to be on the ballot to broaden the debate within the party.

Mr. Corbyn’s victory was then helped by a rule change under which, for just three British pounds, about $4.60, anyone could become a “registered supporter” and take part in the vote. The party, which had around 187,000 members before the May general election, has gained more than 105,000 since, plus 112,000 “registered supporters.” When those affiliated with the party, mainly through unions, are added, more than half a million people had the right to vote, and more than 420,000 ultimately took part.

On Saturday, political opponents moved quickly to condemn Labour’s leftward shift. Michael Fallon, the defense secretary, described the opposition party as “a serious risk to our nation’s security, our economy’s security and your family’s security.”

“Whether it’s weakening our defenses, raising taxes on jobs and earnings, racking up more debt and welfare or driving up the cost of living by printing money — Jeremy Corbyn’s Labour Party will hurt working people,” he said in a statement.

More worrying for Mr. Corbyn will be internal critics. Steven Fielding, professor of political history at Nottingham University, said that the emphatic nature of Mr. Corbyn’s victory made it likely that his opponents would keep quiet for some months, and hope for him to trip up.

“I think there will be some kind of cease-fire, and that they will see how it goes, assuming that in six to 12 months, Labour’s opinion poll position will start to fall, and then they can start acting more assertively,” Mr. Fielding said.

Doing so more quickly might risk the wrath of the party supporters, many of whom have been energized by Mr. Corbyn’s campaign, Mr. Fielding added.

On Saturday they thronged the conference center where the results were announced, some cheering or chanting.

Among those supporting the new leader was Sean Maher, a telecom executive from London, who said he had lost interest in the party during the 1990s because of the policies pursued by Mr. Blair, whom he likened to a Conservative.

“What’s the point of winning elections by being Conservative?” he said. By contrast, Mr. Corbyn, “offers a great deal of hope, and it has been a long time coming.”