Labour's Jeremy Corbyn arrives to take part in a Labour party leadership final debate, at the Sage in Gateshead, England, (Scott Heppell/AP)

Jeremy Corbyn’s stunning transformation from perennial leftist rebel to leader of Britain’s Labour Party upended British politics Saturday and delivered a striking message worldwide: At this anti-establishment moment, parties of the left are just as vulnerable to populist takeovers as parties of the right.

Nearly 60 percent of Labour voters backed him over three more centrist rivals just four months after the party suffered one of its worst-ever defeats in national elections.

The Corbyn victory represented an extraordinary rebuke to Labour’s powers-that-be, especially to former prime minister Tony Blair, who had campaigned vigorously against Corbyn and who argued that his selection would mean the party’s “annihilation.”

But interventions from Blair and other party heavyweights apparently did little to halt Corbyn’s momentum and may have even backfired.

As the summer campaign progressed, the former union organizer evolved from a fringe candidate who barely made it on the ballot to a grass-roots phenomenon who, white-haired and rumpled at 66, stirred the passions of a new generation of Labour activists.

Campaign badges for Jeremy Corbyn. (Ben Stansall/AFP/Getty Images)

Corbyn’s rise echoes that of another senior-citizen socialist who has come out of nowhere this year to rattle his party's center-left establishment. Like Corbyn, Sen. Bernie Sanders (I-Vt.) has been waging a surprisingly effective insurgency in a campaign that was once thought to be unwinnable.

“If you’re Bernie Sanders, you’ll take some heart from this,” said Tim Bale, a politics professor at Queen Mary University of London. “If you’re Hillary Clinton, you’ll be nervous.”

From the campaign trail in South Carolina, Sanders cheered Corbyn’s win, saying he was “delighted” by the news. “We need leadership in every country in the world which tells the billionaire class that they cannot have it all,” Sanders said.

[Bernie Sanders plots a path to Democratic nomination]

While Sanders is still fighting uphill in his effort to outmaneuver Clinton for the Democratic nomination, Corbyn’s once-
quixotic-seeming campaign ended Saturday with a landslide win that would have seemed unimaginable in Britain only months ago.

In a fiery victory speech, Corbyn vowed to combat society’s “grotesque inequality” and make Britain a more humane country.

“We don’t have to be unequal,” Corbyn, looking professorial in spectacles and an open-collar blue shirt, told a crowd of cheering supporters at a London conference center. “It doesn’t have to be unfair. Poverty isn’t inevitable. Things can — and they will — change.”

Corbyn, who is often photographed biking around his posh north London district, pledged to unify the Labour Party’s polarized factions. He may be helped in that task by Tom Watson, who was elected as deputy leader and is seen as a possible bridge between leftists and centrists.

But within minutes of the results being announced, it became clear just how difficult it will be to hold Labour together. At least seven top Labour politicians announced they would not serve in Corbyn’s shadow cabinet, the opposition party’s main vehicle for challenging the government’s policies.

The Conservative Party also unleashed a stinging attack that previewed what will likely be a relentless drumbeat for as long as Corbyn remains leader of the opposition. Defense Secretary Michael Fallon charged that Corbyn’s win had made the Labour Party “a serious risk to our nation’s security, our economy’s security and your family’s security.”

Blair and a chorus of other Labour titans had previously warned that a Corbyn victory would condemn the party — which governed Britain as recently as five years ago and has traded power with the Conservatives for the past century — to a long walk through the political wilderness.

But Corbyn capitalized on a widespread grass-roots backlash against the political establishment that has grown out of the wreckage of the Iraq war and the widening wealth gap since the Great Recession.

[Jeremy Corbyn wants to apologize to Iraq for the 2003 invasion]

Although Corbyn has served in Parliament for more than three decades, he succeeded by portraying himself as a straight-talking political outsider who is unafraid to challenge the powerful, even within his own party.

Corbyn has often bucked the Labour leadership on critical issues — including the vote to authorize the Iraq war — and his message resonated among Labour voters who believe their party has been reduced to a pale imitation of the Tories, especially as it lurched to the center under Blair.

No one could accuse Corbyn of mimicking the Conservatives, and his win is likely to sharpen the ideological divide in the U.K.

He has previously called for Britain to leave NATO, favors unilateral nuclear disarmament and champions the nationalization of vast sectors of the economy, including the railways and the energy industries. He has also said that he will apologize on behalf of Labour for the Iraq invasion and that Blair could face war-crimes charges.

[What the rise of Britain’s Jeremy Corbyn means for the rest of the world]

In his first official act as leader, Corbyn gave an impassioned address to tens of thousands of demonstrators who thronged central London demanding that the government allow in more refugees from Syria and other conflict zones.

“Open your hearts and open your minds and open your attitudes to people who are desperate,” Corbyn exhorted the crowd. He then joined in for a rousing rendition of “The Red Flag,” a socialist anthem.

In the crowd, some Corbyn backers popped champagne and guzzled it in the streets as they waved signs reading “Tories out, refugees in.” Wherever Corbyn went Saturday, he was greeted with a chorus of “Yes, we did!”

“This is a man who is absolutely principled, who is interested in debate about ideas and who doesn’t care what color tie you wear,” said Laura Parker, 45, who leads a children’s charity. “He doesn’t have every single word that comes out of his mouth scripted by somebody else.”

Parker said her sense of elation put Saturday “up there with my wedding day. It maybe trumps it.”

She said she had “lost faith” in Labour years ago but was inspired by Corbyn to rejoin.

The party saw a surge of new supporters this summer, many of whom paid 3­­ pounds — or about $5 — for the chance to vote for the party’s new leader. The ease of joining led to accusations that Labour opponents were joining up just to vote for Corbyn and thereby torpedo the party’s chances of winning the next election. But Labour officials said Saturday that they thought the vote had been free and fair.

Outside Britain, Corbyn’s victory was welcomed by leftist parties. Syriza, the coalition of socialists and communists that faces its own election next Sunday after a tumultuous eight months governing Greece, called the vote “a message of hope to the people of Europe.”

The radical left has also been on the rise in countries such as Ireland and Spain, both of which hold elections in the coming months.

Corbyn’s victory places him in line to be the party’s candidate for prime minister in the next general election, in 2020. But he did not focus on the 2020 campaign in his victory speech, and analysts say it’s more than likely he’ll face an internal leadership challenge before that vote.

The emergence of the radical left as a key player in European politics comes at least several years after the far right began asserting itself. In Britain, the anti-immigration U.K. Independence Party had its best-ever result in May elections, proving that voters on both ends of the spectrum are looking for alternatives to the traditional power-brokers.

“This isn’t just a leftist phenomenon. It’s a populist phenomenon,” Bale said. “It’s the idea that voters are fed up with politics as usual and an elite that’s compromised.”

Read more:

Election may set U.K. on path to a split, isolation

A potential British prime minister wants to apologize to Iraq for the 2003 invasion

Long-shot Miliband now the favorite to become Britain’s prime minister

A new political order in Scotland threatens to upend the British election

Karla Adam in London and
John Wagner in Florence, S.C., contributed to this report.