Taking 59.5 percent of first-preference votes, the long-shot win is seen as a clear renunciation of the policies currently dominating UK and European politics. According to reports, Corbyn sailed past rivals Andy Burnham, who trailed with 19 percent, and Yvette Cooper, who won 17 percent. Liz Kendall, the "Blairite" candidate, came in last with 4.5 percent.
Supporters say the triumph marks a sea-change within the established order.
"British politics will never be the same," said Salman Shaheen of Britain's progressive Left Unity party in a statement. Corbyn's victory, Shaheen added, "shatters the main parties’ consensus on austerity, war and many other issues. This victory is part of a new kind of politics that is rising across Europe, as people reclaim hope and mass movements grow for real alternatives."
In a speech following the win, Corbyn thanked those who, catalyzed by his candidacy, flocked to the Labour party. The number of people who paid £3 to take part in the vote nearly tripled the party's size, as it grew to roughly 550,000 people.
Speaking to his supporters, Corbyn vowed to return the party to its founding principles of "justice" and "democracy," and work to better the whole of the country.
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His policies include increased public spending and nuclear disarmament. During his speech, Corbyn called for Prime Minister David Cameron to show more "compassion" in dealing with the Syrian refugee crisis, adding that he would attend a demonstration planned in London later on Saturday.
His victory, he said, confirms to everyone "that the objectives of our party are intact, our passion is intact, our demand for humanity is intact, and we as a party are going to reach out to everyone in this country to take us on that journey together so no one is left on the side, everyone has a decent chance in life and a decent place in society."
"Thank you in advance to us all working together to achieve great victories," not just electorally, he added, but "for society to show we don't have to be unequal, it doesn't have to be unfair, poverty isn't inevitable, things can—and they will—change."
During his campaign, the north London MP promised that, under his leadership, the Labour party would apologize "to the British people for taking them into the Iraq war on the basis of deception and to the Iraqi people for the suffering we have helped cause."
Greece's leftist Syriza party issued a statement congratulating Corbyn, saying his leadership will significantly strengthen the "pan-European front against austerity," adding that the win "sends a message of hope to the people of Europe." On Monday, Corbyn is expected to join former Greek finance minister Yanis Varoufakis at an event in London to discuss "alternative[s] to austerity, humanitarian disaster and market meltdown."