Leader Jeremy Corbyn pulled out of a planned campaign event as the party began a hunt to find the source of the leak, which saw details of the top-secret document splashed across newspaper front pages.
The draft sets out plans to nationalise key industries and reverse years of austerity with spending on public services funded by tax hikes on the rich and big business.
Tories branded it a recipe for taxes and borrowing which would put the UK on the "road to ruin". But shadow chancellor John McDonnell insisted it was "a modernising, exciting new modern manifesto".
Disarray surrounding the security breach was heightened when a BBC cameraman was injured by a car carrying Mr Corbyn to the meeting to finalise the manifesto.
As the Labour leader's convoy arrived for the "Clause V" gathering of party and union bosses, the car ran over the cameraman's left foot, leaving him needing ambulance attention.
According to the document obtained by the Daily Mirror and Telegraph, a Corbyn government would:
* Renationalise railways as each private franchise expires, with fares frozen and guards put back on driver-only trains.
* Establish publicly-owned bus companies.
* Return Royal Mail to public ownership following the coalition government's "historic mistake" of selling it off.
* Take energy "back into public ownership" by setting up a rival to the existing Big Six private firms.
To pay for the policy pledges, Labour has already announced plans to hike corporation tax to 26% by 2022, bringing in an extra Â£20 billion for the Exchequer, and indicated that people earning more than Â£80,000 will face tax rises.
But the manifesto indicates a further levy on firms "with high numbers of staff on very high pay".
Labour has insisted its manifesto will be fully costed and the document vows to eliminate the deficit and balance the budget by the end of the next parliament.
In an effort to bridge party divides over Trident, it commits Labour to the nuclear deterrent, but in a nod to Mr Corbyn's opposition to the weapons it says "any prime minister should be extremely cautious about ordering the use of weapons of mass destruction which would result in the indiscriminate killing of millions of innocent civilians".
Labour's joint election co-ordinator Andrew Gwynne played down the significance of the leak, which he said had put the party's policy platform at the top of the news agenda.
While its early appearance was "not ideal", he told BBC Radio 4's Today programme: "On the plus side we are all talking about the Labour Party this morning, we are all talking about some of the visions of how this country can be better."
Unite boss Len McCluskey hailed the policies as "really, really exciting".
He predicted voters would welcome a higher minimum wage, the abolition of zero-hour contracts and the nationalisation of the railways, which he described as "the most popular policy out there".
But Conservative Defence Secretary Sir Michael Fallon said the manifesto contained "lots of promises to spend lots more money on lots of different things".
"What they have not made clear is how that is going to be financed, how much more borrowing there's going to be and what the implications are for the economy if you go on a massive borrowing spree like the last Labour government did," he said.
"They would borrow more and that in the end is the road to ruin."
Mr Corbyn's effort to win support from voters who backed Brexit may be hampered by the manifesto's measures on the EU and immigration.
The document rules out setting a target for cutting net migration - something Theresa May has committed to despite so far failing to hit the Conservatives' "tens of thousands" ambition.
Instead, it says Labour believes in "fair rules and reasonable management" of migration, but rules out "making false promises on immigration numbers".
Mr Corbyn has insisted "Britain is leaving the European Union" but the manifesto rules out breaking away from Brussels without a deal.
A senior Labour source said suggestions the manifesto was leaked by Mr Corbyn's team were "categorically and completely untrue", while a separate Labour source denied the leak had come from the party's headquarters, insisting no-one there had the manifesto.
One former Corbyn aide, ex-spokesman Matt Zarb-Cousin, pointed the finger of blame at members of Labour's HQ staff, who he has previously accused of undermining the leadership during his time in the party.
But Labour MPs John Woodcock and Ian Austin, both prominent critics of Mr Corbyn, defended party staff against the "smear", insisting there was no evidence to link them with the leak.