The Football Lads Alliance, the group behind marches against what they call “Islamist extremists”, uses a secret Facebook page full of violent, racist and misogynistic posts, targeting Sadiq Khan and Diane Abbott, as well as playing down the crimes of the Finsbury Park mosque attacker, Darren Osborne.

The Observer has gained access to the FLA’s 65,000-strong Facebook group in the run-up to its planned march “against extremism” in Birmingham on 24 March, which anti-racists fear could be the UK’s biggest ever Islamophobic mobilisation.

The page, which is invite-only and monitored by a team of administrators, states the FLA are “not fascist thugs” but includes posts by members calling for Khan, London’s first Muslim mayor, to be “hanged” and for Abbott, Britain’s first black female MP, to be “run over”. There are also posts claiming mosque attacker Osborne is a “scapegoat” and suggesting he was right to plot to kill Labour leader Jeremy Corbyn.

The findings come after Facebook removed the pages of Britain First, the far-right group, for repeatedly violating its community standards and sending out anonymous anti-Muslim letters to addresses across the country.

Tottenham Hotspur supporter and former hooligan John Meighan, who founded the FLA after the London Bridge terror attack and led a march of thousands of fans through the streets of London last October, insists the group is made up of ordinary supporters opposed to all forms of extremism. But the main speaker in Birmingham is the rightwing former Ukip leadership candidate Anne Marie Waters, who calls Islam “evil” and has links to an ex-member of the far-right British National party.

FLA members posted this month that Khan was “the enemy within” and a “Trojan horse” who should be hanged “as a warning to all”. Others called Khan a “traitor”.

FLA posts include call for London mayor Sadiq Khan to be ‘hanged’. Photograph: Johnny Armstead/REX/Shutterstock

After Osborne was found guilty in February of killing Makram Ali when he deliberately drove his van into a crowd of Muslims, four FLA members commented that Osborne was in fact a scapegoat. Some went even further, insisting he was justified in his plot to kill Corbyn, with FLA supporters remarking that attacking the Labour leader was “right” and “not a bad thing” because he is an “enemy of the UK”.

As one of the highest-profile black women in parliament and a critic of the FLA, Abbott is a regular target on the page. FLA members last month repeatedly demanded that Labour’s British-born shadow home secretary “should be deported” and shared explicit racist memes.

There is a constant stream of sexist and racist references to Abbott’s appearance, with FLA members asserting, without challenge from the administrators, she is “a primate with lipstick”, “fat pig” and “bitch”.

Many of the posts on the page back the former leader of the English Defence League, Tommy Robinson, who attended the FLA’s last march. A recent video of Robinson fighting with a black man in Italy titled “migrant invaders” attracted 413 likes from FLA members, including Meighan. Meighan denied the group was rightwing and said he did not condone racism or sexism. “Anyone who shows any form of racism is expelled straight away. We have moderators who remove people,” he said.

He said he had invited Waters as a specialist on Islamic terrorism. “We are not against moderate Muslims. We are worried about people carrying out extremist attacks,” he said.

FLA members were “frustrated” with Abbott and Khan, Meighan said. He said Abbott was a regular critic of the FLA and claimed she was racist towards white people. “That is why people have taken offence to her,” he said. “It has been proven that she is actually racist herself so you need to [do] a little bit of investigation into that, around white people.”

Diane Abbott, the shadow home secretary, says threats on the page should be taken seriously. Photograph: Yui Mok/PA

Meighan confirmed he had been banned from all football grounds for three years in 2006 but said he was a family man now: “It’s 12 years ago. It’s hardly worth reporting it.”

Abbott said the threats had to be taken seriously, following the murder of the Labour MP Jo Cox by a far-right extremist in 2016: “They are inciting racial hatred. Both Sadiq and myself are black and minority ethnic politicians. It is actually quite frightening ... because we know with Jo Cox that anything could happen.”

Weyman Bennett from Stand Up to Racism, which is planning a counter-demonstration in Birmingham, said the FLA was like the EDL before it morphed into a violent street movement that brought chaos to towns and cities across the country. “They are the EDL mark two, except they are much bigger and have tried to learn the lessons of the EDL. They have hidden a lot of their views. That is why they are in a closed Facebook group.”

Bennett added that the EDL also claimed to be non-racist when it was founded. “When the EDL started off, they said there weren’t a racist group. They even burnt a Nazi flag. But it ended up with them sieg heil-ing, attacking mosques and turning over mixed-race couples.”