ED Miliband has promised to attend a mass protest against the Coalition's austerity programme that union leaders predict will see hundreds of thousands of people taking part in demonstrations in Glasgow, London and Belfast.

The Labour leader, who refused to attend an anti-cuts rally in 2010 but who took part in one last year, was asked during a conference question and answer session if he planned to go to the TUC demonstration in London on October 20. He replied to applause: "Yes, I will be there."

Paul Kenny, General Secretary of the GMB, had earlier told the conference in Manchester that the three-city protest would signal the start of the tide being turned against the UK Government.

Insisting unions were now fighting back to protect public services and workers' rights, he added: "We must not let this rotten coalition destroy the achievements of the past generations. This is the most right-wing government Britain has had in modern times. It is daring to tread even where Margaret Thatcher did not tread."

Mr Kenny explained that despite a confrontation on the Labour leadership's approach to the public sector pay freeze – it said jobs must come before pay rises – the link between trade unions and Labour was now as strong as ever and it was vital they worked together to fight the Government.

Brendan Barber, the TUC General Secretary, said of Mr Miliband's announcement: "It is great news that he is coming. He is very welcome. Our aim is for one nation against austerity."

In his Q&A, Mr Miliband, still riding high from the praise he received for his keynote address on Tuesday, insisted Labour in opposition had "bucked the historical trend so far".

He referred to how Labour oppositions in the past had, by mid-term, "turned inwards, we had taken leave of the electorate and taken leave of our senses".

However, the Labour leader praised the party for showing discipline and not self-indulgence and for deciding to take on the real opposition, which was the Conservative-led Government.

Mr Miliband said that while the party was bucking the trend of previous Labour oppositions, he would be the "eternal warrior against complacency".

He insisted David Cameron was "not a uniter but a divider", a PM who was dividing Britain.

He said: "Because we are a united Labour Party, a Labour Party of the whole UK, we are the people who are best-placed to unite this country. Friends, let's go and do it."