Thousands of Scots children could be being plunged into poverty under tax and benefit changes which start today, Good Friday, campaigners claimed.

One charity accused the UK Government of creating a "cliff edge" for parents with the changes on what has already been dubbed Bad Friday.

The start of the new financial year today triggers a number of measures which the respected Institute for Fiscal Studies (IFS) think-tank estimates will see families with children stand to lose an average of £511 a year.

Satwat Rehman, director of One Parent Families Scotland, said: "This is going to be a shock for the families who lose out. A cliff edge has been created and they will be stunned."

"Families are already struggling to make ends meet, with reduced help with childcare and higher living costs, and now this is really going to hit them."

Jeanette Campbell of Citizens' Advice Scotland said the changes could "push many people into poverty and debt. She added: "Some 20% of Scots children are already living in poverty. These changes will make the situation even worse."

Two changes to the tax credit system – part of a £3 billion cuts programme – will hit households on modest and middle incomes.

Families will lose their working family tax credit, worth up to £3870, if they fail to raise their combined employment from 16 to 24 hours a week. Most of them earn about £17,000 a year.

Households will also lose the child tax credit, worth around £545 a year, if they earn more than around £26,000, down from a previous threshold of £41,300.

The IFS calculates that these two changes alone will affect around 84,000 households across Scotland.

Facing a second day of negative headlines on the issue, the Tory-LibDem Coalition came out fighting. UK ministers said hard decision had been taken, but insisted other measures which take effect today, particularly an increase in the threshold below which workers pay no tax, would mean 15 times as many people would gain.

On the local election campaign trail in Wales, Prime Minister David Cameron said he did not "did not accept" the IFS figures and that overall more than 24 million taxpayers stood to gain more than £5 a week.

Officials said that included the increase in the personal allowance, to £8105, a cash rise in the basic state pension and increases in other benefits, while still taking into account the tax credit cuts.

However, official figures from the House of Commons library, released by Labour last night, suggested the true figure was just 81p a week. That gain was also wiped out for families with children because of a freeze in child benefit. Those with one child will lose £1.15 a week.

The SNP's Work and Pensions spokeswoman Dr Eilidh Whiteford said: "These changes will cause severe hardship for working families, and plunge tens of thousands of households into poverty. After delivering a Budget tax cut for millionaires, the Tories and, without a word of protest, their Liberal Democrat partners are cutting support for vulnerable families."

Labour said the withdrawal of working tax credits in particular meant some families stood to lose 90 times as much as they would gain from the Coalition's policies.

Shadow Chancellor Ed Balls has accused the Government of for hitting families with a "tax credit bombshell".

He added: "I don't think the Government is taking into account the other changes coming in today, the freeze in child benefit, the cuts to tax credits which mean that a family with children is worse off, according to the IFS, by £511, and a million families lose all of their tax credits entirely."