Alec McFadden was dozing in his armchair when a loud bang on his front door brought him to his senses with a jolt. Looking out of the window of his Wallasey home, he saw a young man half slumped in the driveway. "I couldn't see his face but he looked like he was in some sort of trouble, like he needed help," says McFadden. "I opened the door just a bit to ask if he was OK and he threw himself at me and started hitting me around the head."

What McFadden did not realise at the time was that he was not being punched but stabbed. "I think it went on for a couple of minutes before I managed to get the door closed. I turned round and my daughter was screaming. It was only then, as I put my hand to my face and felt the blood, that I realised what had happened."

The attack, which left the long-time union activist with serious injuries, was the latest and most violent incident in a campaign of intimidation that has been waged against opponents of the far right in the UK over the past five years. Like hundreds of people who have spoken out against the rise of the British National Party and other extremist groups, McFadden's picture and home address have been collected by far right activists and posted on a website called Redwatch.

The site, which has links with the neo-Nazi organisation Combat 18 and a host of European fascist organisations, is hosted in the US but registered and run from the UK. It lists the personal details and shows the photographs of anti-racists - many taken during protests against the British National Party - alongside the slogan: "Remember places, traitors' faces, they'll all pay for their crimes." This month a delegation of MPs and union activists will visit the Home Office to call for the site to be closed down. It is a familiar refrain and in the past officials have argued that because the site is hosted abroad, there is nothing they can do. However, Redwatch's sister site in Poland, which was also hosted in the US, was recently closed down after collaboration between authorities in the the two countries, and Home Office minister Vernon Coaker has agreed to champion the campaign within government.

Redwatch was launched in 2001 and takes its name from a Combat 18 newsletter produced in London in the 1990s. For the first few years it was just another online talking shop for hardline racists and fascists, offensive and unpleasant but apparently not dangerous. However, in April 2003, those behind the site signalled that Redwatch meant business. Leeds school teachers Sally Kincaid and Steve Johnson had been involved in local campaigns against the BNP and other far-right groups for years. Then their personal details appeared on Redwatch following a demonstration they had attended in the Pudsey area of the city. A couple of weeks later they suffered a fire-bomb attack at their home, which left their car burned out.

The incident was a turning point. Those featured on Redwatch were no longer being subjected to threats and harassment but to physical attacks. In the months that followed, journalists, politicians and local anti-racist activists were listed on the website. Among those targeted was Peter Lazenby, a journalist on the Yorkshire Evening Post, whose picture now adorns the front page of the site. He has been a long-time opponent of the far right, and has won awards for his reports on the BNP, which gained its first councillor in Leeds in May.

"In some ways being on Redwatch is recognition that as a journalist you must be doing something right," says Lazenby. "But my overriding feeling is one of anger and resentment that these people feel free to try and intimidate me and my colleagues and threaten us with violence. There is a personal impact on your life. When it first happened, I didn't take much notice - I'd been getting threats of one kind or another from far right groups since I started reporting on them in the 1970s. But then they got hold of an old address of mine and I thought that wasn't fair on the people who had moved in, so I warned them and contacted the police."

West Yorkshire officers took the threats seriously and advised Lazenby to improve his personal security, telling him to vary his route home from work. "I have been on that site since it started and, apart from verbal attacks in the street, nothing has happened to me yet. But every time you hear of an attack it really makes you think. A young man whose details went up after he was spotted delivering anti-fascist leaflets was badly beaten. Then there was the stabbing [of McFadden] over on Merseyside, and there are plenty of other cases that don't make the papers. Each time an attack happens, it pulls you up. But you can't let it influence your work, you have to keep going."

The threat to reporters has grown steadily since Redwatch was launched. In most cases it follows a similar pattern. A newspaper reports on the activities of the BNP or other far right groups, then journalists' details appear on the site, and threats and intimidation follow. According to the National Union of Journalists, reporters who have published "fair, accurate and professional stories" on the BNP and other far right organisations have been targeted in Leeds, Liverpool, Sheffield, Sunderland, Birmingham and Cardiff. The website even has a new section dedicated to "red journalists".

Campaigns of intimidation are also being waged against local anti-racist activists. "We sometimes find that, although a lot of people are worried about the far right in their area, they don't want to get involved because they are concerned about intimidation," says Nick Lowles of anti-fascist organisation Searchlight, "and that is due in part to Redwatch and the threats and violence that come with it."

The racists and fascists behind Redwatch are not deterred by the campaign to close the site. A notice on the homepage reads: "Due to a hysterical campaign by Marxist moaners against Redwatch, we use several domain names and multiple servers for this site." It adds: "While it is time to be legal we must stolidly endure whatever the puppets of the capitalist state see fit to inflict upon us, and when it is time to revolt we must be prepared to unleash all the furies of hell."

Sympathisers claim Redwatch is little more than an act of self-defence, insisting that they are only doing what anti-fascist campaigners who monitor the activities of rightwingers have been doing for years. But Lowles says this is a dangerous myth: "This is absolute rubbish. There is no anti-racist equivalent to this site. There is nothing that lists home details of fascists and certainly nothing that encourages attacks on them. Redwatch is not an act of revenge but something altogether more sinister. It is designed to intimidate and harass anti-racists and anti-fascists to the point where the individuals targeted no longer campaign against fascist groups. It is political intimidation and classic fascism."

The British National Party, under its leader Nick Griffin, has tried to distance itself from Redwatch - the blatant intimidation and violence sits uncomfortably with its attempt to portray itself as a mainstream, democratic party. But despite official denials, some anti-racist activists say their pictures have been taken by members of the BNP only to appear on Redwatch a few days later. "I was campaigning against the BNP before the local elections," says Carl Morphett from Kirklees in West Yorkshire, which now has three BNP councillors. "We were photographed by two BNP members in Yorkshire and a day later we appeared on Redwatch. There is no doubt that the BNP uses this site to try and intimidate people - to suggest anything else is ludicrous."

The BNP strenuously denies the claim. Yesterday a spokesman for the party said: "We are not involved, we have absolutely nothing to do with Redwatch at all." He added that, as far as he knew, no individual members had taken pictures that had subsequently appeared on the site. "If they are doing so, they should not be doing so."

Six months after the attack on the Leeds schoolteachers, an investigation by the Guardian and Searchlight shed light on the true nature of Redwatch, uncovering a secret hitlist of targets, including social workers, journalists and politicians. Only a handful of known neo-Nazis had access to the secure email network that listed the names and addresses of targets as well as plans for attacks on anti-racists in their homes or during public meetings. One subscriber, who called himself Mole Intelligence Bureau, wrote: "Redwatch has accumulated many names and addresses, along with pictures of the targets, many of whom have had nothing done to them. Now's the time to start a proper campaign of violence and intimidation towards those who seek to see us silenced or imprisoned for our beliefs."

One of the targets was Lazenby and numerous addresses were posted on the site for members to "check out". One message read: "We need to find this reporter fast. If we can scare this cunt off, then we might get an easier time instead of being slagged off and made to look a bunch of muppets." In another message, posted by a BNP supporter from Batley, the people behind Redwatch are asked if they will be attending an anti-racist meeting in Dewsbury in June. The event, which was addressed by Leon Greenman, a Holocaust survivor, was described as a "Holohoax meeting". One respondent advised: "The best place to attack the reds [is] just after the meeting finishes as they are walking to catch their buses or going for their cars."

The network listed dozens of people "for further research", including the divisional police commanders for Dewsbury and Huddersfield, the chief executive of Kirklees Council, the director of a West Yorkshire health authority and housing officers. For many anti-fascists this was final proof that Redwatch represented a serious threat. Known neo-Nazis with violent criminal pasts were planning to step up their campaign of intimidation and were planning attacks against specific targets. The evidence was passed to the then Home Secretary David Blunkett and officials declared that action was imminent. But after examining the details, the Home Office again said that because the site was hosted in the US there was little they could do - listing public information online is not a crime and the website is full of disclaimers.

Following an initial meeting in August with a delegation of MPs, trade unionists and anti-racists, Coaker agreed to champion the cause. According to Home Office officials, he is in discussion with senior police officers, and contact has also been made with the US authorities to see if it is possible to take joint action. That appeared to come a step closer recently when it emerged that a new legal opinion published in the US argues that the site is not protected under the first amendment. In a separate development, anti-racist campaigners say they have identified the main Redwatch organiser and have passed his details to the police.

But while the authorities on both sides of the Atlantic "look again", the intimidation goes on. Since the knife attack, McFadden has received a letter and a phone call warning him that if Redwatch is closed down his children will be shot. Liverpool Liberal Democrat councillor and anti-racist campaigner Robbie Quinn received death threats against him and his family after his details were posted on the site, as did singer Alun Parry, who has been targeted for performing at a refugee benefit concert.

Angela Eagle, who is the MP for Wallasey, McFadden's home constituency, and whose photograph also appears on the front page of Redwatch, argues that the website represents an unacceptable threat to people's democratic right to free speech. "The powers are available to take action against this site. What we desperately need now is the political will." Lazenby says there is growing frustration that Redwatch is still up and running. "I feel a degree of anger against the authorities that they have done nothing about this site. They have said repeatedly that there is nothing that can be done, but I simply don't believe them. We have quite rightly managed to close down paedophile sites and others that are deemed unacceptable, and I am sure that if Redwatch was targeting the richest 100 people in the country, it would be swiftly dealt with".