Jeremy Corbyn once spoke at a rally attended by members of Al-Muhajiroun
The revelation will fuel claims that the Labour leader's track record points to him being "soft" on Britain's enemies if he becomes Prime Minister this week.
The pro-Palestinian event attended by Mr Corbyn took place in 2002, in London's Trafalgar Square.
Among the crowd were supporters of the militant group Hezbollah and up to 300 members of the Al-Muhajiroun group, reported the Marxist Weekly Worker paper at the time.
Jeremy addressed a broad-based rally in support of Palestinian rights
Some held placards reading "Palestine is Muslim" and members of the noisy contingent chanted "Skud, Skud Israel" and "Gas, gas Tel Aviv", and abused a female Israeli peace activist.
Al-Muhajiroun has been a banned terrorist organisation since soon after the 7/7 bombings in London in 2005.
It has been linked to attacks including the 2013 murder of Fusilier Lee Rigby.
It was led by radical Islamists Omar Bakri Muhammad, currently believed on the run in Lebanon, and Anjem Choudary, who was jailed last year for encouraging people to support and join so-called Islamic State.
The group has been linked to attacks including the murder of Lee Rigby
Alleged London Bridge attack ringleader Khuram Butt is believed to have been a supporter of the group.
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There is no suggestion Mr Corbyn – whose party has faced accusations of tolerating anti-semitism under his leadership – did anything wrong at the 2002 rally.
A spokesperson for him said today: "Jeremy addressed a broad-based rally in support of Palestinian rights.
"It was a public event and he was in no way responsible for the views of all of the thousands of attendees.
"Jeremy condemns Al-Muhajiroun in the strongest possible terms."
Prime Minister Mrs May defiantly insisted that Britain's democratic process will not be derailed by the bloody attacks in Manchester and London during the campaign.
Asked at an election rally in Stoke on Trent if voters worry about the possibility of further attacks on polling day, she said: "The national threat level is at severe, which does mean that a terrorist attack is highly likely. It has been severe for some time. People have been planning, ensuring people’s safety on that basis.
"People should go out there to vote. I want to see people going out to vote because I think that is a very important message, that our democracy will not be deterred."
Theresa May insists that Britain's democratic process will not be derailed
She brushed aside claims that security service failures allowed the recent attacks but promised any lessons would be learned and extra powers given to the services and police as needed.
She added: "The police and Security Service have done a good job in foiling a number of plots – five in the last three months and a significant number in the last few years as well."
Vowing to crack down on terrorism on the internet, she added: "I think the companies should accept their responsibility in relation to what is being put on their platforms, because we see hateful ideology being spewed across their platforms by the extremists.
"That can lead to terrorism, and we don't want to see a safe space online for terrorists to plane their attacks. Se we will continue to put pressure with others on those companies.
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"But we're also very clear we do need to look at regulation for cyber space because we need to ensure there is no safe space for terrorists online."
She sidestepped calls to apologise to the families of those affected by the attacks for failing to keep them safe, saying: "What matters for the future is making sure we have in place a government that is willing to support our police when they do what they need to do.
"I support shoot-to-kill – eight minutes and our police officers had taken those terrorists out. Jeremy Corbyn doesn't."
For the Conservatives, Defence Secretary Sir Michael Fallon said: "Jeremy Corbyn wants to be Prime Minister in just two days but this latest revelation about his association with extremists shows exactly why he is unfit to lead the country.
Michael Fallon said Mr Corbyn has a long track-record of siding with people who want to damage UK
"He has a long track-record of siding with people who want to damage and attack the UK and there can be no excuse for his decision to address this rally.
"The choice on Thursday is between Theresa May, who acts to protect our national interest and keep our country safe, and Jeremy Corbyn, who stands up for the very people who want to do us harm.
"There is a very real risk that Jeremy Corbyn and his shadow Home Secretary Diane Abbott will be in charge of our security and borders on Friday unless people vote for Theresa May and her Conservative team."
Mr Corbyn in 2009 introduced members of Hezbollah and Hamas as "friends" at a meeting, voted against a law to ban various terror groups including Al Qaeda shortly before 9/11, boasted about opposing all anti-terror legislation and joined a meeting where dead IRA terrorists were honoured.