The British prime minister said Labour "cannot be trusted" and predicted that the plan to impose VAT on private schools would cause "chaos" for families.
Meanwhile, Labour said Starmer's first steps would be restoring economic stability and cutting NHS waiting lists as "the work of change begins".
The Conservatives made a series of claims about the impact of Labour's announced policies and speculation about further measures Starmer could introduce.
Keir Starmer used his final Saturday of the campaign to hold a celebrity-laced rally in London. (AP PHOTO)
They claimed up to 134,000 children in private schools could be forced into the state sector by September, although the 2018 report on which this assertion is based points to that figure as a possibility over five years.
The Conservatives also pointed to a series of tax rises which Labour has not ruled out, but which are not party policy, along with speculation about other measures such as a Europe-wide youth mobility scheme and granting votes to EU citizens, which are also not Labour policies.
The attacks come as the Conservatives battle to close Labour's opinion poll lead, which has remained at around 20 points.
"It's clear that Labour would do irreversible damage within just 100 days of coming to power," Sunak said.
"Whether it's announcing a suite of tax rises or throwing thousands of families' plans for the autumn term into chaos, with children wondering if they will have a desk at school to go back to.
"Labour would throw open our borders with their illegal migrant amnesty and free movement for under-30s in their deal with the EU, making us the soft touch migrant capital of the world.
"They cannot be trusted. We must not surrender our taxes, our borders and our security to them. Only the Conservatives will deliver tax cuts, a growing economy and a brighter, more secure future for everyone."
In a UK Sunday Telegraph interview, Sunak added that Labour's tax plans would "bankrupt people in every generation".
Labour's national campaign co-ordinator Pat McFadden argued that it was the Conservatives who had already delivered chaos, and his party which offered the hope of change.
"The country faces a big choice on Thursday. If you vote Tory on Thursday - or don't vote at all - nothing will change.
"The NHS won't get any better. The Tory manifesto threatens another instalment of Liz Truss. The doom loop of low growth and high tax won't change.
"The Tory Party itself won't change - the lessons from Partygate, dodgy COVID contracts and the insider betting scandal won't be learned.
"Chaos will continue, and they'll continue to put their own interests before the country's."
Sunak, a Hindu, spent part of the final weekend of the campaign visiting the Neasden Temple.
On the Saturday night visit he said he felt he had the community's "blessing", adding: "I'm determined to never let you down."
Starmer used his final Saturday of the campaign to hold a celebrity-laced rally in London, with messages of support from Elton John and Game Of Thrones star Kit Harington.
