Jeremy Corbyn has defended his decision to adopt a neutral stance in a future Brexit referendum as "a sign of strength" and "maturity".
The Labour leader told a BBC Question Time leaders' special on Friday that he would not campaign for Leave or Remain if his party wins power.
Prime Minister Boris Johnson questioned how Mr Corbyn could be "indifferent" on such a vital issue.
And Lib Dem leader Jo Swinson accused him of an "abdication of leadership".
But Mr Corbyn said acting as an "honest broker" was a "sensible way forward".
If he becomes prime minister, Mr Corbyn has promised to negotiate a new Brexit deal with the EU and put this to a public vote alongside the option of Remain.
However, in Friday's programme he confirmed he would maintain a neutral stance in this referendum before carrying out the result.
The BBC Question Time Leaders Special
Speaking on a campaign visit to Sheffield on Saturday, Mr Corbyn said: "I think being an honest broker and listening to everyone is actually a sign of strength and a sign of maturity.
"Our country has to come together - we can't go on forever being divided by how people voted in 2016.
"My role as the Labour prime minister would be to ensure that [another referendum] is carried out in a fair way, that the offers put are fair, and that I will carry out the result of that referendum.
"I think this is actually a sensible way forward that can bring people together."
What have the other parties said?
Health Secretary Matt Hancock said Mr Corbyn's stance "won't wash" with voters, adding that the Labour leader had "decided to be indecisive" on the "critical question of our times".
"You can't ask the public to be prime minister of this country when you can't even make your mind up on Brexit," he told the BBC.
Speaking during Friday's Question Time special, Mr Johnson said the Labour leader's ambition to negotiate a new Brexit deal would be undermined by his neutral stance.
"I don't see how he can do a deal when he is going to be neutral or indifferent," he said.
During a campaign visit in west London on Saturday, Lib Dem leader Jo Swinson said Mr Corbyn's position was "astonishing".
"On the biggest issue faced for a generation, they are not going to take a position," she said.
"Remainers in this country need a leader, not a bystander."
Speaking in Leven, Fife, SNP leader Nicola Sturgeon said Mr Corbyn was showing a "woeful lack of leadership" on Brexit.
Brexit Party leader Nigel Farage also accused Mr Corbyn of a "failure of leadership" on "the defining issue of our day".
What else is happening on the campaign trail today?
While visiting the Design Museum in Kensington, Ms Swinson also defended the Liberal Democrats' policy of stopping Brexit altogether if they win the general election.
The Lib Dem leader was challenged over the party's policy to revoke Article 50 by both Leave and Remain voters in Friday's Question Time special.
She said: "If we go from 20 Lib Dem MPs to 326 Lib Dem MPs that will be a democratic mandate.
"That will be a big big boost for the Liberal Democrats and I think people will recognise that will be a seismic event."
"If we don't we're going to keep campaigning for a people's vote to stop Brexit," she added.
Away from Brexit, Mr Corbyn has unveiled Labour's "youth manifesto" - called The Future Is Ours - at a rally in Loughborough.
The document includes:
- Giving 16-year-olds the right to vote
- An end to tuition fees and the return of maintenance grants
- Free bus travel for under-25s
- A ban on unpaid internships
- An additional £250m to build up to 500 new youth centres
- 3,500 qualified counsellors to guarantee every child access to school counsellors
Earlier he met workers outside an Amazon depot in Sheffield, where the Labour leader promised to end zero-hour contracts.
He also vowed to force multinational companies to pay their fair share of tax in the UK, with a promise to prevent them offshoring tax bills to "lower tax regimes".
An Amazon spokesperson defended the company's record saying: "The government wrote the tax laws and they are designed to encourage investment, and we are investing heavily in creating jobs and infrastructure across the UK."