Theresa May was confronted by a fan of Jeremy Corbyn's manifesto as she took calls from the public in a "telephone town hall" event.
But she dismissed the Labour document, released earlier in the day, as "a very long wish list" which did not add up.
Thousands of voters listened in to the conference call event as the Prime Minister was quizzed for an hour on issues ranging from Brexit to tax to education and knife crime.
One caller, named Jane, accused Mrs May of "sending attitudes to disabled people back to the 1940s" due to the Government's replacement of disability living allowance with personal independence payments.
Another, called Jim, suggested that she might face more of a challenge than expected in the June 8 general election after the launch of Labour's manifesto.
"Today's manifesto from the Labour Party set out a lot of information that to the vast majority of the population in this country will appear very attractive," Jim told Mrs May.
"I thought, with things like the minimum wage, the NHS, schools and so on, unless you are very careful you are going to find a significant swing to Labour on those issues."
Mrs May responded: "From what I see, it is a very long wish list, a whole lot of promises, but if you look at it, it doesn't add up.
"The question you always have to ask about the Labour Party is how will they pay for it?
"I think it's ordinary working people who pay the price."
The Prime Minister went on the attack over Labour's plan to nationalise water, saying: "One thing I know is that nationalising the water companies is not a way to ensure that bills will be kept down and the service will be good."
She defended the Conservative Government's record on disability benefits, telling Jane: "It is about focusing funding on those who are most in need and helping those who can move into the workplace to do so."
Mrs May declined to rule out tax rises if the Conservatives win the June 8 poll, but said it was clear from Mr Corbyn's manifesto that Labour's "natural instinct" was to raise taxes, while Tories were a low-tax party.
She also dodged a question on bus passes for the elderly, telling Shirley from Cheltenham that she would have to wait for the Tory manifesto later this week to see if they were being kept.
Members of a Conservative mailing list and followers of the party's Twitter feed had been invited to take part in the event, and it was clear at least some of those putting questions to the PM were supporters who had taken her key election messages to heart.
One praised her "strong and stable" leadership, another urged her to tell Scottish First Minister Nicola Sturgeon that now was not the time for a second independence referendum and a third said his main worry was voters would believe media reports that she was heading for a landslide and stay home on polling day.
Others were keen to persuade Mrs May of the merits of their own policy ideas.
John, 72, from Bedfordshire suggested she should divert some of the aid budget into support for owners of diesel cars like himself to switch to electric models, made in the UK.
David told the PM the Blue Badge parking scheme should be extended from people with physical disabilities to include those on the autistic spectrum.
Patricia's concern was that politicians refused to state that immigration was the "main reason" for NHS queues and school place shortages.
Mrs May said she recognised that there was an impact from immigration not only on public services, but also on wages and jobs and that as home secretary she had worked for six years to try to establish control on numbers.
She told another caller concerned that she might accept compromises in the upcoming Brexit talks: "Don't worry, I'm going to be batting for a good deal."
Moderator Nick Ferrari picked around 20 callers to make their points to the Prime Minister, but said there were "literally thousands" more listening in and hoping to ask a question in the first event of its kind in the campaign.