In a robust start to his party conference in Manchester, Mr Miliband pledged to restore the scrapped 50p tax rate if there were a General Election tomorrow.
He also told the UK's banks they had three years to separate their high-street and investment arms or as leader of the next Labour Government he would do it for them.
And he hit back at unions, warning Labour was also the party of the "private" sector.
Mr Miliband's efforts to show he is strong enough to lead the UK came as he moved to bolster the position of Scottish Labour leader Johann Lamont last night in the face of SNP fire over her controversial benefits review.
Describing her as "our excellent Scottish leader", he was due to tell the conference's Scottish reception she had been "right" to start a debate "about how we deliver social justice when money is tight". He said: "We can't pretend that there aren't difficult decisions we're going to have to make," he was expected to say as he rounded to attack the Scottish Government. "The truth that the SNP must accept is that if nothing is done, it will be the most vulnerable who suffer."
Mr Miliband also warned about the very real possibility of the break-up of the UK, saying: "Scotland could become independent. It's up to the Labour Party. No-one's going to win this battle except us."
Mr Miliband was forced to deny over the weekend he needed to change to appeal to voters, following consistently poor personal poll ratings.
However, he struck a markedly more assertive note as he warned Labour would not be ruled by the unions and insisted he would do things his "own way".
Labour is keen to use the conference to cast Mr Miliband as the next occupant of Downing Street. Polls show he is struggling at just the same time as his party builds a strong lead over the Conservatives.
Party insiders believe the public has struggled to see Mr Miliband's personality for a range of reasons, including the personal drama that surrounded his leadership battle against his brother David and the accusations that he is too geekish.
Earlier, the Labour leader rejected criticism of his leadership style saying he would "keep doing it my own way".
But he suggested he himself wanted voters to see a different side of him.
"People want to know about you - about who you are and what makes you tick. And that's what I'm going to do," he said.
On the eve of the conference the Unite union leader Len McCluskey had threatened to kick out the "New Labour cuckoos" and reclaim the Labour Party. But Mr Miliband hit back saying Mr McCluskey was wrong to oppose a public-sector pay freeze which his party insists has saved jobs.
Mr Miliband did nothing to ease tensions by adding that Labour under his leadership would be "the party of the private sector" as much as the public sector and unions were not "pulling the strings".
He also issued a strong warning that the next Labour government would not give in to union demands on public spending. "We are not going to have lots of money to spend," he said, as Ed Balls, his Shadow Chancellor, warned that the party "could not promise to put everything right straight away".
Mr Miliband also said he had a "very clear message" for the UK banks. "Either they sort out themselves so that once and for all the high-street bank is not an arm of the casino operation, or the next Labour government will, by law, split those banks up."
His pledge on the 50p tax rate was condemned by business leaders. John Cridland, director general of the CBI, said: "The UK must be open for business and a 50p rate would put entrepreneurs off from coming and investing in jobs here."
A Coalition Government spokesman accused Mr Miliband of making "opportunistic" promises and "chasing headlines".
The unions also hit back, increasing pressure on the party over public-sector pay.
They warned the party would lose the next election with a "watered-down" version of the Government's policies.
They also attacked Mr Balls, drawing up a dossier of what they said were "Balls Ups" during the last Labour Government. GMB leader Paul Kenny said he would read out the list at a fringe event at the conference today.
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