The 66-year-old socialist, whose policies have been compared to those of Greece’s Syriza and Spain’s Podemos, was named leader after clinching 59.5 percent of the 422,664 votes cast by Labour party members and supporters.
The newly-elected leader condemned “grotesque levels of inequality” and “an unfair welfare system” in his victory speech to party members in central London.
The veteran campaigner also called for Britain’s Conservative government to show more “compassion” in dealing with the Syrian refugee crisis and said he would attend a demonstration planned in London later on Saturday.
He said Labour was “united and absolutely determined in our quest for a decent and better society that is possible for all”.
North Islington MP Corbyn has electrified Labour’s leadership race, which was triggered by the resignation of Ed Miliband after he lost May’s general election to David Cameron’s right-wing pro-austerity Conservatives.
“The fightback now of our party gathers speed and gathers pace,” Corbyn added in his speech.
‘Jez we did’
The veteran MP comfortably beat the three other more centrist candidates in the race — Yvette Cooper, Andy Burnham and Liz Kendall — who all had far stronger support from fellow Labour MPs.
“Jez we did!” his delighted supporters chanted in the conference room in reaction to the decisive outcome.
Corbyn has spent a total of 32 years on Labour’s backbenches and only won a handful of votes from his own MPs — but was swept to victory on a wave of enthusiasm from Labour supporters following the end of the Miliband era.
The stunning result has however sparked talk of a deep split within Labour — which was thrown into turmoil by Cameron’s shock election win in May.
Corbyn’s triumph echoed anti-austerity sentiment across Europe because of his rhetoric about ending austerity cuts and taxing the rich.
His policies include spending more on public services like schools and hospitals, scrapping nuclear weapons, renationalising industries like the railways and involving Islamist groups Hamas and Hezbollah in Middle East peace talks.
“Jeremy Corbyn has won more than 50 percent of the votes in this round and I am therefore delighted to declare Jeremy Corbyn elected as leader of the Labourparty,” said Jim Kennedy, chair of Labour’s National Executive Committee, unveiling the result.
Cheering supporters welcomed the news wearing red “Team Corbyn” T-shirts reading and waving signs saying “I Voted For A New Kind of Politics”.
Corbyn drew strong support from students who had never voted before to older people disillusioned with Labour since it moved to the centre-ground of British politics under Tony Blair in the 1990s and led the country into the controversial 2003 Iraq war.
The result was widely anticipated with bookmakers having slashed the odds of a Corbyn victory from 100/1 outsider to 1/7 favourite.
The bearded, grey-haired vegetarian has crammed in 99 campaign appearances, eschewing soundbites and usually wearing sandals and carrying a cup of tea.
The MP was plastered over Saturday’s newspapers wearing a stripey cream T-shirt and navy blue shorts, alongside trainers and socks.
In a positive omen for Corbyn, lawmaker Sadiq Khan — who nominated the veteran left-winger for the Labour leadership — was elected as the party’s 2016 London mayoral candidate on Friday.
Khan, who did not actually vote for Corbyn, comfortably beat Blairite former Olympics minister Tessa Jowell, boosting media speculation over a landslide for the Islington North MP.
Back to 1980s?
Popular right-wing tabloid The Sun described Khan’s “shock victory” as part of the “Jeremy Corbyn revolution” — but warned the victory could spark a deep split in the party ousted from power in 2010.
“A victory threatens to plunge Labour back to the 1980s, given his hard-left views on nationalisation, nuclear weapons, tax and business,” it wrote.
The left-wing Guardian also warned that some frontbenchers were unlikely to serve on Corbyn’s team due to “fundamental” differences that would “destabiliseLabour policies”.
But former mayor of London Ken Livingstone described Corbyn as a “consensual” politician who wanted proper discussions about the direction of the Labour party on key issues ahead of the next general election in 2020.
“It will work if Jeremy starts to connect with people,” Livingstone said.
Despite the “Corbynmania” of his grassroots campaign, Tony Blair — Labour’s most electorally successful leader who is now deeply unpopular over Iraq — has warned that his victory would split Labour and consign the party to electoral oblivion.