SNP depute leader Stewart Hosie accused Chancellor George Osborne of “acting like Robin Hood in reverse”, while both parties fear more than 500,000 children in Scotland whose families receive tax credits will suffer.
There are 296,000 households in Scotland with children who receive tax credits, with much of the cash going to help working families on low wages.
It is widely anticipated that Mr Osborne, delivering his first Budget in a Conservative administration after five years in coalition, will cut the payments.
Mr Hosie, the SNP Treasury spokesman at Westminster, said: “The Chancellor has set himself the task of tackling the UK’s productivity weakness in his first true blue Tory Budget.
“I hope that any plans he announces are more successful than his record on deficit reduction, in which he totally failed to meet any of the targets he set himself.
“But according to the leaks so far, the reality is that George Osborne is more concerned with deeper cuts to public services and reducing people’s incomes - acting like Robin Hood in reverse.
“The likely changes to be announced for tax credits will simply cut household budgets, which as the First Minister has pointed out will hit the poorest families with children hardest.”
He added: “The Chancellor has said that his plan is designed to achieve the fiscal targets set out in the Charter for Budget Responsibility.
“However, we know the scale of the spending cuts the Chancellor has planned - as set out in the March Budget - significantly exceeds what is actually required to meet the UK Government’s own fiscal targets.
“There is, therefore, flexibility for the UK Government to meet its fiscal objectives without implementing in full the spending cuts that are currently planned for the coming years. However, the Budget looks set to be an offering from an austerity cult - damaging the economy, and hurting hard working and vulnerable people.”
Labour’s shadow Scottish secretary Ian Murray said: “The Tories came to office claiming that this Government would be on the side of working families. Before the Budget has even been published it is becoming clear that, once again, it is working people who will bear the brunt of this Government’s cuts.
“With their proposed cuts to tax credits, the Budget risks cutting the feet from under people who do the right thing, go out to work every day and try to do the best by their families.
“Nearly 300,000 families in Scotland, including half a million children, will be affected by George Osborne’s cuts to tax credits, at the same time as he refuses to rule out a further tax cut for millionaires.
“This Government needs to be tackling the root causes of the rising welfare bill - low pay and rising housing costs - to bring down the deficit in a sustainable way.”