RMT union general secretary Bob Crow dies
General secretary of the Rail Maritime and Transport union Bob Crow has died aged 52
In a statement on its website, the union said he died in the early hours of Tuesday morning.
London Mayor Boris Johnson, with whom he clashed over plans for the Tube, said: "I am shocked. Bob Crow was a fighter and a man of character."
Mr Crow was elected general secretary of the RMT in 2002 following the death of former leader Jimmy Knapp.
The BBC's political editor Nick Robinson said Mr Crow "was, some argue, the most successful union leader in terms of securing jobs and pay for his members".
Only a day before his death, he told BBC Radio 4's PM programme that MPs deserved a pay rise.
'Sad day' Mr Johnson said in a statement: "Whatever our political differences, and there were many, this is tragic news."
"Bob fought tirelessly for his beliefs and for his members.
"There can be absolutely no doubt that he played a big part in the success of the Tube, and he shared my goal to make transport in London an even greater success.
"It's a sad day."
Former mayor Ken Livingstone told Sky News: "Young people will learn lessons from him and realise he stood up for his members."
He added: "He was great company."
Born in 1961 in east London, Mr Crow got his first job on the underground at the age of 16, fixing rails and cutting down trees by the track.
He became a local representative for the then National Union of Railwaymen at the age of 20.
Last month, Mr Crow joined his members on the picket line during a Tube strike, called in protest at the mayor's plan to close ticket offices.
Manuel Cortes, leader of the TSSA rail union, which also took part in the strike, said: "Bob Crow was admired by his members and feared by employers, which is exactly how he liked it.
"It was a privilege to campaign and fight alongside him because he never gave an inch."
TUC general secretary Frances O'Grady said he was "an outstanding trade unionist, who tirelessly fought for his members, his industry and the wider trade union movement".
Paul Kenny, GMB general secretary, added: "Even people who didn't like what he did agreed he did it very well."
Kevin Maguire, associate editor at the Daily Mirror, said on Twitter: "Bob Crow was a big burly bloke with a great brain, wit, heart of gold and surprisingly soft handshake. Missing him already."
A lifelong Millwall supporter, Mr Crow was not a member of any political party, and was often equally critical of Labour - especially New Labour - as he was of the Conservatives.
Nevertheless, many Labour MPs gave glowing tributes to him.
John McDonnell, convenor of the RMT parliamentary group, wrote on Twitter: "In Bob Crow we have lost one of the finest trade union leaders and socialists our movement has known. I am devastated by this tragic news."
His colleague Jeremy Corbyn said Mr Crow was also "a very dedicated opponent of racism and fascism whenever it reared its head", as well as taking part in anti-war campaigning.
BBC News - RMT union general secretary Bob Crow dies
