Acting Labour leader Harriet Harman has accused the government of not "being straight" with people about the Budget.
She said the PM had promised that families on less than £40,000 would not lose tax credits - but the Budget "small print" suggested otherwise.
She also said pensioners would be "worse off" and would not see any increase in their pension next year.
David Cameron said Labour MPs had not suggested "one penny piece of one saving" to help cut the deficit.
Small print
Ms Harman was speaking at prime minister's questions, a day after Chancellor George Osborne announced a rise in VAT from 17.5% to 20%, a two-year pay freeze for public sector workers earning over £21,000 and substantial welfare savings.
She said Mr Osborne had promised to be open about how the Budget would affect people but details in the "small print" showed "changes to eligibility criteria for tax credits that could see families on a combined income of £30,000 lose all their tax credits.
Mr Cameron said the government was "making sure that the least well-off families get the most money" and said the measures would not increase child poverty.
Ms Harman also challenged the prime minister to say whether pensioners would be better off as a result of the Budget - and claimed the coalition government had not "set aside one single penny for this big promise to pensioners" - to link the state pension to earnings from April 2011.
And she said: "Next year prices are due to go up more than earnings so bringing forward the earnings link by a year doesn't give pensioners anything extra... we all know they will pay more in VAT".
