The Tories claim 94% of working households are better off under the tax and benefit changes coming into effect.
Labour’s Ed Balls claimed average families were £1,100 a year worse off since 2010, including Monday’s changes.
Both parties say their rivals have secret plans to raise taxes if they win the general election.
The Lib Dems, meanwhile, are angry with the Conservatives for claiming credit for an increase in the personal income tax allowance, which they say they had to force Tory ministers to accept.
The personal allowance – the amount someone can earn before they are taxed – has gone up from £10,000 to £10,600.
It is due to go up to £12,500 by 2020, but senior Lib Dem David Laws said his party “would want to go far faster than the existing trajectory for increasing the personal tax allowance”.
In other election news:
Lib Dem Treasury Spokesman Danny Alexander’s claim in the Independentthat a Conservative minister – whom he does not name – told a Lib Dem cabinet colleague: “You take care of the workers and we’ll take care of the bosses” has been dismissed as “total rubbish” by a Downing Street source
Lib Dem Scottish Secretary Alistair Carmichael confirmed the memo which claimed SNP leader Nicola Sturgeon would prefer David Cameron to be prime minister was written in the Scotland Office
Shadow chancellor Ed Balls, speaking in Leeds, said Britain could not afford five more years of the Tories.
“Families are £1,100 a year worse off on average – that is the true cost of a Tory chancellor,” he said, citing independent figures from the Institute for Fiscal Studies which he said supported his case.
He said the IFS figures took into account all of the changes David Cameron and his government had introduced since May 2010 up until Monday, including the change to the personal allowance.
The IFS “also exposes how families with children have been hit hardest”, he added.
“Their damning conclusion is that low-income households with children lose the most as a percentage of their income from changes implemented by the coalition.
“While millions are paying more, we know that millionaires are paying less.”
This election issue includes income tax and national insurance levies and business taxes.
He repeated Labour’s claim that a Conservative government would increase VAT to make their sums add up – something denied last month by David Cameron.
The Conservatives claim Labour would be forced to increase National Insurance contributions to pay for their spending plans.
One of the biggest changes coming into force on Monday is the relaxation of pension rules, so that those with a defined contribution pension “pot” can take out as much as they like when they reach the age of 55.
Liberal Democrat Pensions Minister Steve Webb said the principle was to trust people with their own money – but he urged people to seek advice and not rush into any decisions.
Labour’s pensions spokesman Gregg McClymont welcomed the “flexibility” in the new system but voiced concerns the pensions industry was “finding it difficult to adapt so quickly to such a big change”.
Other changes coming into effect include a higher limit on ISA savings, the Marriage Tax Allowance, a 2.5% increase in the state pension, the abolition of employer National Insurance on under 21-year-olds and the scrapping of the 10p tax band on savings. Working age benefit increases have also been capped at 1%.
The Conservatives have produced Treasury figures suggesting 94% of households will be better off under the changes, with 92% of pensioner households also gaining. The figures do not include workless households.
In a speech, Prime Minister David Cameron will say: “Today is a big day for our country. It’s ‘money-back Monday’ – a day when, quite simply, hardworking taxpayers get to keep more of their own cash.”
He will add that as a result of Conservative action – including raising the personal allowance – “our country becomes a better and fairer place to live… where those who put in, get out; where hard work is rewarded; and people are trusted.”