Ed Miliband has become the first Labour Party leader to address the Durham Miners' Gala since Neil Kinnock in 1989.
Organisers say the 2012 Gala - the largest remaining working class demonstration in the country - drew crowds upwards of 100,000.
Trades union members from around the country crammed into the old Racecourse to hear Mr Miliband speak.
The Conservatives have criticised him for "cosying up" to Labour's left-wing union paymasters who they claim secured his leadership.
Marchers gathered behind around 80 banners, with more than 50 brass bands playing - more than in the 1960s and 1970s when the Durham Coalfield was still in operation.
Mr Miliband listed some of the Labour heroes who have spoken before him at past galas, including Keir Hardie, Clement Attlee, Harold Wilson and Barbara Castle.
"I am proud to follow in their footsteps," he said. "I am proud to be here today."
He used his speech to launch an attack on the "divisive" policies of the Tories in government.
He said: "One rule for those at the top and another rule for everybody else. They cut taxes for millionaires and they raise taxes on pensioners.
"It's business as usual in the banks and small businesses go under.
"They try and divide our country between rich and poor. Between north and south.
"Same old Tories. Not building for the future but ripping up the foundations. Not healing our country, but harming it. Not uniting our country, but dividing it."
Conservative Party co-chairman Baroness Warsi said his appearance marked a return by Labour to the politics of the 1980s.
She said: "By breaking 23 years of silence from the Labour leadership at the Durham Miners' Gala, Ed Miliband is handing his party back to Kinnock.
"Red Ed is using the Durham Miners' Gala to cosy up to his militant, left-wing union paymasters. He's still driving the Labour Party away from the centreground of British politics."
Dave Hopper, secretary of the Durham Miners' Association which organises the meeting, said it was only right the Labour leader attended.
He said: "All those jokers before him should have come too.
"In a county that is totally Labour-controlled and where every MP since time immemorial has been Labour, Blair, Brown and Kinnock refused to come.
"It was an insult to the voters in the county."
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