Harriet Harman went to war with the Daily Mail on Tuesday over its claims she turned a blind eye to a paedophile lobbying group in the 1970s. She aggressively demanded an apology for the tabloid's "smears" and tweeted pictures of its website showing a 12-year-old girl in a bikini.

The Labour deputy leader said there was absolutely no reason to say sorry for working for the National Council for Civil Liberties (NCCL), after the Mail published a series of stories about the group's links to the Paedophile Information Exchange (PIE) between 1975 and 1983.

Stepping up her attack on the Mail, Harman said the "vile" PIE had nothing to do with her work at the civil liberties organisation and called on the newspaper, edited by Paul Dacre, to retract its "politically motivated" claims that she was an apologist for child sex abuse.

"I'm not going to apologise because I've got nothing to apologise for," she told ITV. "I very much regret that this vile organisation, PIE, ever existed and that it ever had anything to do with NCCL. But it did not affect my work at NCCL.

"They had been pushed to the margins before I actually went to NCCL and to allege that I was involved in collusion with paedophilia or apologising for paedophilia is quite wrong and is a smear."

Harman said her husband, Jack Dromey, successfully fought to stop PIE having any influence within the NCCL in 1976 – two years before she joined the organisation as its legal officer.

Documents seen by the Guardian appear to back up her claim that the NCCL executive banned PIE from bringing a motion complaining about the harassment of paedophiles. A PIE leaflet from the time criticised the NCCL executive's typical "oppression of minorities".

Patricia Hewitt, a former Labour health secretary, who was general secretary of the NCCL from 1974 to 1983, has not commented on any of the allegations.

Dromey, who was on the NCCL executive from 1970 to 1979, has said he was at the forefront of repeated efforts to condemn PIE. He described the Mail's accusations as "beneath contempt".

However, on Tuesday night, Tom O'Carroll, the former chairman of PIE who was jailed in 1981 for "conspiring to corrupt public morals", challenged the idea he was sidelined from 1976. He said PIE was tolerated in the NCCL because it was among the "radical and liberal forces" in the organisation that the ambitious politicians could not afford to alienate.

"Harman has been quoted saying that after 1976 they were pushing back against PIE and were trying to exclude the organisation from any kind of influence at NCCL, but that is news to me because I sat on the NCCL gay rights subcommittee from, I think 1976, for at least two or three years," he told the Guardian. "I ceased to be chair of PIE when I was prosecuted for corrupting public morals in mid-1979. My presence was never challenged, I was always made very welcome including by other members of the committee."

O'Carroll, who wants the age of consent for penetrative sex lowered to 12 and believes that younger children should be able to consent to other sexual activity that is not likely to be physically harmful, said he understood Harman and Hewitt had been privately opposed to PIE's affiliation to the NCCL, but says they never formally objected to his presence on the subcommittee.

"They are now talking about 'campaigning to kick people out' and how 'we rejected them', but at the time they didn't really do very much at all," he claimed.

But an aide to Harman said it was still the case that PIE had absolutely no influence on the leadership, especially after the showdown in 1976, and there were "many, many subcommittees" at the NCCL.

Other NCCL members from the time utterly reject the idea PIE had any status or influence within the organisation. A statement of the group's priorities from 1981 shows it was focused on prisoner rights, freedom of assembly, right to know, women's rights, reform of police complaints, gay rights and Northern Ireland emergency legislation.

Anna Coote, who was on the NCCL executive in the 70s, said paedophile rights were never an issue on the agenda. "I can't tell you how little anybody thought about this. PIE was completely on the sideline. It did not have any status or influence," she said. "There was an AGM where a couple of blokes in dirty macs turned up and wanted to move some motion, but we went to some lengths to make sure it wasn't moved."

Labour sources said party leader Ed Miliband was "100%" in support of Harman's decision to take on the Mail, having already said on Monday he "set no store" by the stories.

Harman focused her counterattack on the Mail website's publication of images of girls in swimwear. She tweeted a link to a Mail Online picture of a 12-year-old in a bikini and gave her support to a petition.

Asked if the Daily Mail promoted indecency, Andrew Pierce, its consulting editor, said it was a "family newspaper" and had not published photographs of anything that could not be seen on a beach.

He said the newspaper accepted that Harman abhorred paedophilia but argued that the Labour figures still had questions to answer about why PIE was allowed to remain an NCCL affiliate until as late as 1983.

Previously, the Mail group has dismissed the suggestion it is smearing Harman, saying it is "a newspaper's job to ask awkward and controversial questions – questions that in this instance are still awaiting a satisfactory answer".

Other politicians leapt to Harman's defence, including shadow attorney general Emily Thornberry, who said the story was ridiculous, and the Tory backbencher Nadine Dorries, who said there was no way that Harman would have condoned paedophilia.

Tom Watson, a senior Labour backbencher, tweeted that reducing the story to a row between Harman and the Daily Mail "misses the point: kids were abused by members of PIE and investigations thwarted".

• This article was amended on 26 February 2014. An earlier version said Tom O'Carroll wanted "the age of consent dropped to just four". That was a proposal put forward by the Paedophile Information Exchange. O'Carroll's personal view is that the age of consent should be 12 for penetrative sex.