Pamela Geller is on a mission to save the free world and she's doing it, on this occasion, in a bikini as she writhes around in the sea.

"Here I am in my chador, my burka," Geller jokes to the camera in one of a string of video blogs campaigning against Islamic "world domination" shortly before kicking back in the waves. "There is a serious reality check desperately needed here in America and I'm here to give it to you, but I'm just not ginormous enough. What can I say? And on that note I'm going to go swimming in the ocean, and visit my mama, and fight for the free world."

This strange performance might suggest that Geller is a figure consigned to the margins of the widening and increasingly heated debate about the role of Muslims in America. Far from it.

The flamboyant New Yorker, who appears on her own website pictured in a tight fitting Superman uniform, has emerged as a leading force in a growing and ever more alarmist campaign against the supposed threat of an Islamic takeover at home and global jihad abroad – and never more so than in the present bitter dispute over plans to build an Islamic centre near the site of the World Trade Centre, brought down by al-Qaida.

Geller has been at the forefront of drumming up opposition to the centre, two blocks from Ground Zero, through an array of websites such as the Freedom Defence Initiative (FDI) and Stop Islamisation of America (SIOA). They have become increasingly influential as conservative politicians exploit anti-Muslim sentiment before November's congressional and state elections.

SIOA is behind a series of advertisements opposing the "Ground Zero Mega Mosque", as Geller calls it, which appeared on the sides of New York buses this week picturing a plane flying into one of the World Trade Centre towers and a mosque divided by the question: Why Here?

Geller's answer is that the planned centre is viewed by Muslims as a "triumphal" monument built on "conquered land".

As extreme as that may seem, Geller and her views have been embraced by leading politicians such as Newt Gingrich, the former Republican speaker of the US House of Representatives, and John Bolton, the conservative former US ambassador to the UN, who are scheduled to speak at a rally against the controversial New York Islamic centre organised by Geller for September 11.

Gingrich this week likened the planned centre to putting Nazi signs outside the Holocaust museum.

The campaign against the centre also has the backing of Liz Cheney, daughter of the former vice-president and prominent conservative activist in her own right.

But while Geller has inserted herself into mainstream politics in America, she has also aligned herself with far-right causes across the globe including the English Defence League in Britain, white supremacists in South Africa and Serbian war criminals.

Geller says that after the September 11 attacks she "began to immerse herself in gaining a full understanding of geopolitics, Islam, jihad, terror, foreign affairs and the imminent threats to our freedoms that the mainstream media and the government wouldn't cover or discuss".

Civil rights groups have accused Geller of "hate speech" for her repeated warnings of a looming threat of "Islamic domination", including a claim that Muslim groups in America are working to impose sharia law on the entire population, and her assertions that the 9/11 attackers were practicing "pure Islam".

Geller has also compared the proposed mosque to a building a Ku Klux Klan shrine next to a black church in Alabama.

But she vigorously denies she is hostile to Muslims. "I'm not anti-Muslim. That's a slanderous slur and it's unfair," Geller said this week. "Secondly, I'm not leading the charge [against the Islamic centre near Ground Zero]. The majority of Americans – 70% – find this deeply insulting, offensive. To call it anti-Muslim is a gross misrepresentation and to say that I'm responsible for all this emotion, again a gross misrepresentation."

Geller, a former associate publisher of the New York Observer, is often found in the professional company of Robert Spencer, a bestselling author who is less generally visible but is taken more seriously as a scholar among conservatives.

Spencer, who describes himself as a consultant to the US military, the FBI and the government's joint terrorism taskforce, is the author of several books, including Stealth Jihad: How Radical Islam is Subverting America without Guns or Bombs. He also runs a high-profile website, Jihad Watch, which helped raise some of the tens of thousands of dollars to pay for the New York bus poster campaign.

Together the pair launched several organisations including the FDI, which says it is fighting "specific Islamic supremacist initiatives in American cities" and hunting down "infiltrators of our federal agencies", and SIOA, which calls itself a human rights organisation and is tied to a similar group, Stop Islamisation of Europe, which goes by the motto: "Racism is the lowest form of human stupidity, but Islamophobia is the height of common sense".

One member of the board of the Freedom Defence Initiative is John Joseph Kay, who has written that all Muslims are out to kill ordinary Americans: "Every person in Islam, from man to woman to child may be our executioner. In short, that there are no innocents in Islam ... all of Islam is at war with us, and that all of Islam is/are combatant(s).(sic)"

Geller and Spencer wrote a book, The Post-American Presidency: The Obama Administration's War on America, for which Bolton provided the forward.

Geller writes for an Israeli media network based in the occupied territories that is the voice of the Jewish settler movement and runs another website, Leave Islam Safely, which claims to offer guidance on how to escape the religion without being killed.

But her principal outlet is her blog, Atlas Shrugs, named after the philosophical novel by the arch-conservative Russian emigre, Ayn Rand, which promoted "the morality of rational self-interest".

In Atlas Shrugs, Geller lays bare her sympathies with extremist groups across the globe. She has vigorously defended Slobodan Milosevic, the former Serbian president who died while on trial at The Hague for war crimes, and denied the existence of Serbian concentration camps in the 1990s.

She has allied herself with racist extremists in South Africa in promoting a claim that the black population is carrying out a "genocide" of whites.

The website also carries a picture of Geller hugging Geert Wilders, the far-right Dutch politician who advocates banning the Qu'ran and the construction of new mosques, and runs a support campaign for him as he faces trial for incitement to hatred.

Geller has also spoken out in favour of the English Defence League. When the anti-Islamic organisation was planning a rally outside parliament earlier this year, she wrote: "How I wish I could be there to stand with the English Defense League".

Geller has claimed regular contact with the EDL leadership and recently published a screed by the organisation's spokesman, Trevor Kelway. She said in one of her blogs: "I share the EDL's goals ... We need to encourage rational, reasonable groups that oppose the Islamisation of the West and not leave it solely to fringe groups like the BNP."

Geller has also said the EDL is misrepresented. "The EDL is routinely smeared in the British media, as the Tea Party activists are smeared in the US media ... There is nothing racist, fascist, or bigoted about the EDL," she wrote.

While mainstream politicians in Britain and other parts of Europe generally steer clear of the likes of the EDL, Wilders and Serbian war criminals, Geller is providing a bridge between foreign extremists and prominent politicians in the US.

Wilders is scheduled to appear on stage at the September 11 anti-mosque rally alongside Gingrich, Bolton and Gary Berntsen, a candidate for the US Senate.

The Southern Poverty Law Centre, the most prominent hate monitoring group in America, said that the campaign against the Islamic centre near Ground Zero had mixed political exploitation with hate-mongering.

"The politicians and other opportunists are stoking the fires," said Marc Potok, who heads the centre's operation to monitor the extreme right. "The politicians are in it because they want to win more seats. The Pamela Gellers of the world apparently will do anything they can to attack Islam and this Islamic centre has provided them with a very large opening."

Potok says that Geller and others have crossed the line from legitimate debate.

"I think we have seen a great deal of hate speech. It is one thing to talk about the sensibilities of New Yorkers and of survivors and relatives of those who died.

"It is quite another to talk about conspiracies on the part of Muslims to dominate the United States, plots to insert sharia law into American statute books, and the idea that Islam is in of itself a great evil. Those things seem to be clearly over the line and we're hearing more and more of that," he said.

Geller did not respond to requests for an interview. But the American Civil Liberties Union, which has spoken out forcefully in support of the right to build the Islamic centre and mosque, said that Geller and others campaigning against the centre were equally protected by the constitution.

"Just as religious liberty is a core American value so too of course is free speech," said Daniel Mach, director of the ACLU's freedom of religion programme.

"It's clear that many are exploiting this issue and the deep-seated anti-Muslim bigotry that underlies much of this controversy for bare political gain [but] there certainly is a constitutional right to speak out against this or any other project.

"We have a robust protection of free speech in this country including the right to speak hatefully."