Children are being crammed "like sardines" into "super-sized" school classes due to seven years of failed Tory education policy, Jeremy Corbyn has said.
Mr Corbyn highlighted Labour analysis of Department for Education figures which showed more than 40,000 primary school children were taught in classes of 36 or more pupils in 2016, up from around 38,500 the previous year.
A total of 16,655 primary pupils were being taught in class sizes of 40 or greater while nearly half a million were being taught in classes containing between 31 and 35 children, the figures showed.
Furthermore, there were 109 primary schools with more than 800 pupils in 2016, compared with just 16 in 2010 when the Tories came to power.
Mr Corbyn said Conservative cuts to education budgets would only make the problem worse.
"Seven years of Tory failure and broken promises have left our schools in a terrible state," the Labour leader said.
"Hundreds of thousands of our children are paying the price, crammed into classrooms like sardines.
"The Prime Minister herself has said that super-sized classes are proof of a school system in a crisis. And that's what we've got on the Tories' watch.
"School leaders and teachers have said that Tory cuts to school budgets will mean class sizes will be forced to grow even larger.
"We cannot risk our children's education in this way.
"Labour will stand up for all children by building a schools system for everyone, keeping class sizes down and making sure schools and teachers have the resources they need to ensure that every child, whatever their background, has access to a world-class education."
In secondary schools there were nearly 305,000 pupils in classes with more than 31 pupils, 18,000 of which were in classes with 36 or more.
Shadow education secretary Angela Rayner said the Tories' "unjustified fixation" with free schools would not solve the problem.
It comes after the Government approved more than 130 new free schools this month.
The Tory general election manifesto can be expected to put more flesh on Theresa May's flagship policy of more grammar schools for England.
"This situation is becoming unsustainable - too many children are being taught in classes which are simply too big," Ms Rayner said.
"The system for school place planning is broken.
"The Tories need to let go of their unjustified fixation with free schools, but instead they have relaxed the rules so even more can be built in areas where there is no demand for places.
"Free schools are clearly not addressing the growing pressures on schools.
"Theresa May has no answers to the big challenges facing Britain, just more of the same failed policies which will take the country backwards and make us more unequal."
A Conservative spokesman said: "This is a massive own goal from Labour. In Wales, where they administer schools with the Lib Dems, the numbers of infants in large classes has risen by 18% in just three years.
"Of course we are not complacent about the situation in England. There is more to do and that's why we are spending a record amount on schools - something we can afford to do because of our careful management of the nation's finances.
"The biggest risk for our schools and our children is throwing away the economic progress we have made. That's what would happen if we had Jeremy Corbyn as prime minister, propped up by a coalition of chaos with the Lib Dems and SNP"
In a stump speech on a campaign visit to Swindon, Mr Corbyn highlighted Labour's promises to ban zero-hours contracts and increase the minimum wage to £10 an hour.
The Labour leader said: "This is the opportunity of a lifetime to challenge the inequality and injustice that exists in Britain today.
"We have had seven years of a Tory Government that has slashed local government spending, slashed our health care spending, created a social care crisis across our whole country and diminished the chances and opportunities of so many of our children for the future.
"This election is about what happens to this country. It is about what happens to the people of this country going forward.
"Are we going to accept the idea that the biggest corporations and the richest people bear the least burden and make the most profit and become the richest?
"Or do we as a country and we as a party put a message out there that it doesn't have to be like this, it can be better, it can be fairer and it can be a more just society?"
During a visit to the Brentry and Henbury Children's Centre in Bristol, Mr Corbyn told reporters he was determined to take the fight to the Conservatives and win the election.
"Within a few hours of this election being called, 2,500 around the country joined the party and many more since," he said.
"We are a party of half a million members and tomorrow in every town and city across the country we are going to be out campaigning on 'Flying Start Saturday' to this campaign.
"This is a party absolutely determined to take the fight to the Tories about inequality and injustice in this country.
"That's the message and that's the unity of our party now."
Mr Corbyn was accompanied by Lesley Mansell, Labour's candidate in the West of England Metro Mayor election on May 4.
The party leader said: "Since Tuesday the number of activists on the streets of Bristol knocking on the doors has trebled, the number of members is increasing, the unity is there.
"We are out to win this election, to deal with poverty, to deal with unemployment, to deal with housing, to deal with social injustice in Britain and bring in a minimum wage that means something.
"Members mean activists, members mean commitment, members mean enthusiasm, members mean determination and members mean getting the message out on every doorstep and I am really excited about it."
During the visit to the children's centre, Mr Corbyn met staff and youngsters and read them the children's favourite We're Going On A Bear Hunt.