Last Sunday's Observer claimed to expose how "an officer from a secretive unit of the Metropolitan police" worked "undercover among anti-racist groups in Britain, during which he routinely engaged in violence against members of the public and uniformed police officers to maintain his cover".

Despite this sensationalist introduction, "Officer A" does not describe his involvement in any violent incidents. No wonder. The organisation he infiltrated, Youth Against Racism in Europe (YRE) is a peaceful organisation of young people, which in the 1990s organised mass protests against racism and the BNP.

A supporter of the Militant Tendency (now the Socialist party), I was the elected national secretary of YRE between 1992 and 1996 and remember YRE often facing violence from the far right, and unfortunately also from the police.

Ludicrously, the article says Officer A's "key success" was to discover that the 1993 demonstration against the BNP's headquarters was going to be "far larger than thought". YRE repeatedly told the police that the demonstration, following four racist murders within two miles of the BNP HQ, would be huge. Fifty thousand attended. We wanted to march peacefully past the BNP bunker. The police response, as the Observer's film shows, was to refuse to allow us to march at all, and to carry out an incredibly brutal attack on peaceful young people using their democratic right to protest.

We now discover that the police not only used violence against our demonstrations but also carried out a secretive, unaccountable and clearly expensive infiltration operation. They gained nothing from it. Far from being secretive we publicly advertised our events – the police could have read our leaflets and newspapers, or attended our public meetings, to find out what was going on.

Nor is there any basis to Officer A's suggestion that he succeeded in making our work ineffective. In fact he had no negative effect on YRE, which was very successful, playing an important role in completely marginalising the BNP for over a decade.

Hackney activists from the time remember Officer A of the Special Demonstration Squad (SDS), or "Peter Daley" as we knew him, as hardworking. When he disappeared in mid-1997, saying he was moving to Greece, he suggested we sell the contents of his flat to raise funds for the cause.

More seriously, they also recall that Peter did not fully agree with our position on how to defeat the BNP. We explained that defeating racist and fascist groups is a political task which required patient campaigning in working-class communities, rather than street fighting. Peter wasn't as convinced of our position as he could have been and tended to argue for brawling with the BNP. Was he sent in partly as provocateur?

The Observer's revelation is not unique. Christopher Andrew's The Defence of the Realm: the Authorized History of M15, published last year, also describes state infiltration of Militant, the National Union of Miners and others.

This is not just of historical interest. As recent articles in the Guardian have revealed, surveillance of peaceful protestors has mushroomed. Police brutality also, as the tragic death of Ian Tomlinson showed, is not a thing of the past.

Today a new generation are becoming involved in campaigning against the BNP, such as Youth Fight for Jobs, which demonstrated in Barking last weekend. At the same time the current economic crisis sees many workers and young people moving to the left. New "Peters" will undoubtedly be sent to infiltrate anti-racist campaigns and left-wing organisations to try and cut across protest. They will not succeed. But questions about whose interests the police act in, alongside demands for them to be made democratically accountable, and for the disbandment of the SDS, the Territorial Support Group and all other similar units, will be an important aspect of future campaigns.