Labour "will defeat" the government's plans to expand grammar schools in England, shadow education secretary Angela Rayner has said.
In a speech to the party's conference in Liverpool, Mrs Rayner said selection by ability "entrenches division and increases inequality".
"No child should be left out or left behind," she said.
PM Theresa May has said the ban on new selective schools has been in place too long and has held many pupils back.
She says 1.25 million pupils are currently in schools which are "failing, inadequate or in need of improvement" and that the government's plans will give every child the high-quality education they deserve to enable them to go as far as their talents allow.
Grammar schools are state secondaries whose pupils are selected by examination at age 10 to 11. There are currently about 163 in England - out of some 3,000 state secondaries - and a further 69 in Northern Ireland.
Under the government's plans, grammar schools would be allowed to expand and all schools in England would be given the right to apply to select pupils by ability.
'Selection is toxic'
In her first major speech since being promoted in July, Mrs Rayner said Mrs May had produced "no evidence" that grammar schools boosted social mobility, and pledged to fight the proposals "with every breath in my body".
"Selection - or segregation as it should be called - entrenches division and increases inequality," she said.
And she accused the prime minister if "telling fairy tales" about social mobility and opportunity.
"Selection is toxic. It tells a clever child they are stupid, strips a child of self-esteem and embeds inequality. Every child has potential. Every child can succeed. No child should be left out or left behind.
While former Labour PM Tony Blair had talked about "education, education, education", she said Mrs May wanted "segregation, segregation, segregation".
"And our Labour Party will fight it, starting on Saturday when we launch our nationwide campaign against more grammar schools.
"We're going to take the fight to the Tories and I appeal to all my Labour colleagues to help lead this fight. Because together, we can defeat this," she told the party's conference.
Under the slogan "education, not segregation", Labour says it will be contacting one million residents of marginal seats in towns and cities such as Swindon and Norwich to highlight the party's opposition to expanding grammar schools.
The campaign will also focus on the increase in university student fees and the scrapping of student grants.
Ms Rayner also announced plans for a taskforce, chaired by Unison official Liz Snape, to "transform early years provision for every family in the 21st century".
The shadow education secretary said every parent should have the right to quality, affordable childcare and told the party that its mission must be to ensure no child's life chances are blighted by lack of support.
She drew on her own personal experience to argue the closure of 800 Sure Start Centres since 2010 had "shut the door in the faces of our children and their parents".
"I left school at 16, pregnant, with no qualifications," she said. "Some may argue I was not a great role model for today's young people. The direction of my life was set. "
"But something happened. Labour's Sure Start centres gave me and my friends, and our children, the help and support we needed to grow and develop. They changed the lives of three million children and their parents."
"Unlike the Tories, Labour will never turn our backs on our children and their families."
Ms Rayner also said Labour would bring back Educational Maintenance Allowance for students in further education and maintenance grants for low and middle income students.
Elsewhere on the third day of the conference, Sadiq Khan congratulated Jeremy Corbyn on his re-election and said the party must focus on getting into government, not internal battles, in his first speech to conference since his election as London mayor in May.
"Labour in power. Not just talking the talk, but walking the walk too. Never sacrificing or selling out on our ideals but putting them in action every single day," he said.
Mr Khan - the party's most powerful elected official in England - said Labour must use its success in London and its strong representation on councils across England as a springboard back to national government.
Meanwhile, shadow health secretary Diane Abbott vowed to "renationalise" the NHS by removing private provision and curbing the internal market which she said had "ripped" patients off.
And deputy Labour leader Tom Watson will launch an independent commission into the future of work, to look into the challenges of new technology and automation.