The pandemic-induced uplift is to be scrapped this week, leaving those receiving the benefit with £20 less per week.
In response, Chancellor Rishi Sunak announced on Monday at his party’s annual conference in Manchester a £500 million investment to help people into work.
Opponents have criticised the UK Government, with the first ministers of Scotland, Wales and Northern Ireland writing to Prime Minister Boris Johnson this weekend.
Speaking during a visit to Glasgow’s Queen Street Station on Monday, the First Minister said: “This UK Government and the Chancellor, I don’t know if they think people are just daft and can have the wool so easily pulled over their eyes.
“The cut to Universal Credit, which is due to take effect later this week, will take £20 a week from the poorest families in the country, leave some parents unable to feed their children or heat their homes.
“That is a cut worth £6 billion across the UK – £6bn being taken out of the pockets of the lowest income families across the country – and yet the Chancellor stands up and expects some thanks about an announcement of half a billion pounds.
"It in no way compensates for that morally indefensible cut to Universal Credit that he’s about to preside over.
“Over the weekend I and the first ministers of Wales and Northern Ireland came together, an unusual move for all of us, to join together as one to say to the Prime Minister and the Chancellor ‘do not make this cut, because you’re literally taking the food out of children’s mouths’. It is unconscionable, it is indefensible.”
In an interview with STV News, the Prime Minister said: “What we’re saying is that the way forward for this country is not to raise taxes and subsidise low pay and not to keep going with the massive £407bn Covid package.
“The way forward is to support people.”
Meanwhile, Scottish Tory leader Douglas Ross accused the First Minister of being “out of touch” with working families.
During a fringe meeting at the Tory conference, Mr Ross said: “We are the party of working-class unionists in Scotland now because we represent their values.”
Responding to the Tory leader, Ms Sturgeon said: “I’ve just been re-elected as First Minister with a historically high share of the vote including in working class communities across Scotland, so perhaps let the facts and the reality speak for themselves.
“The shame of these Tories – they are about to take food out of the mouths of children in working class communities the length and breadth of Scotland, including in Douglas Ross’s own constituency, and they have the nerve to make comments like this.
“Maybe Douglas Ross would like to come with me and I’ll introduce him to some working-class communities across the country and then he’ll see who’s in touch with them.”
The Tory leader urged the First Minister to “name the time and place” for her proposed visit.
“I’ll go with Nicola Sturgeon to one of the working-class communities devastated by the drug death crisis that has spiralled out of control on her watch,” he said.