Unite general secretary Len McCluskey went on the offensive minutes after the opening of the TUC Congress in Brighton.
"We are asked by the media whether our Congress is relevant," he declared from the rostrum.
"Let us say loudly: 'Yes‚ - it is to give confidence to working people so that we can rise like lions and make our roar heard'."
Warning that trade unionists would be scapegoated by Prime Minister David Cameron, Mr McCluskey said: "Cameron's reshuffle is all about placating rightwingers and it's more than likely that he'll throw them an anti-union bone."
But he said trade unionists were ready to rise to the challenge.
"As we defend our rights and our communities we are not going to break the law as a gesture.
"But we are no longer going to let the law paralyse us."
He was speaking in favour of a motion which listed the Tory hitlist enshrined by its Beecroft report and the "far-right ideologues" of the party's Trade Union Reform Campaign.
Mr McCluskey urged delegates to enshrine it as a TUC policy, which would seek to commit Labour to give British workers the level of rights enshrined in international standards.
He said: "This government is able to build on the betrayal of Labour who failed to deal with the crisis in trade union rights in any serious way in their 13 years in power. This is a stain on the Labour Party. It must stand for the rights of labour.
"It's no good trying to deal with the legal chains that bind us, with leaflets and hopes for a different government in the future alone."