When should people suspected of committing a crime in the course of exercising their right to public protest be prosecuted? That is the question the director of public prosecutions (DPP), Keir Starmer QC has set out to answer. But his guidelines to the Crown Prosecution Service (CPS) threaten our right to protest, which is protected under the Human Rights Act 1998, particularly in view of questionable police practices at public demonstrations.

The decision to prosecute a case is based on whether the CPS considers there is a realistic prospect for conviction as well as whether prosecution is in the public interest. According to the new guidelines, certain factors make it "more likely" that prosecuting a protester will be considered to be in the public interest.

Starmer's hope is that his guidance will help prosecutors "differentiate between violent or disruptive offenders" and those "whose intent was … peaceful".

He mistakenly assumes that people come to public protests with either peaceful or violent intentions. In fact, "violence" often flows from police "crowd-control" tactics at demonstrations, such as kettling, the use of batons, agents provocateurs and undercover policing, all of which have been deployed with increasing frequency in recent years. This is particularly problematic because, in court, defendants cannot invoke "the atmosphere of the day" in explaining their behaviour.

According to the DPP, "prosecutors should have particular regard to whether … a person had come to the protest equipped with … items that could be considered body protection" as this "may indicate … anticipation of disorder". In light of the widespread use of police batons, protesters who wish to exercise their right to voice dissent whilst also protecting themselves should not face a higher risk of prosecution by doing so.

The idea that suspects "in possession of a weapon at the time of the offence" should face a greater likelihood of prosecution raises the question of what is to be considered a "weapon". The category of "weapon", as opposed to "offensive weapon", is so broad as to cover practically anything. My shoe could be a weapon if I beat someone with it. A term so broad contravenes the principle of legal certainty. Clarity is crucial in order for citizens to be able to regulate their behaviour. Is a drinks flask a weapon? Or a placard? The point is we don't know.

In July last year, Frances Fernie, 20, was sentenced to 12 months in a young offenders' institution for throwing two placard sticks, which hit nobody.

Under the DPP's guidelines, prosecution is less likely where an act was committed "in the heat of the moment". Fernie argued in court that his behaviour was "a moment of hot-headedness" and the judge accepted that he had been "caught up in the heat of the moment". As Defend the Right to Protest has argued, Fernie should never have been prosecuted. It is crucial to get the decision on prosecution right because for protesters, a conviction is the end of life as they know it.

The guidelines indicate that where suspects take "steps to conceal their identity", prosecution will be more likely. Not only does this seem ridiculous if we consider mid-winter demonstrations – I myself have wrapped a scarf around my face when demonstrating in cold weather – but it also raises serious concerns in view of police tactics. The systematic police practice of videoing public protests and releasing stills to the media means it is scarcely surprising that demonstrators might wish to hide their identities. Worse, when individuals, confronted with their image in the media and keen to clear their names, come forward, they are swiftly arrested and charged with public order offences. Such retroactive policing was common following the riots last summer and the student demonstrations of 2010.

These individuals then find it difficult to defend themselves in a political climate that is becoming more hostile towards dissent.

Given police tactics of baiting protesters, it is not clear why "significant damage" being "caused to property" should make prosecution more likely. At a student demonstration in November 2010, the police parked a van amid protesters. The police must have known that frustrations brought about by their crowd-control techniques would be directed against the abandoned vehicle. In view of such police tactics, damage to property in the course of public protest is not necessarily a sound basis for considering prosecution of protesters to be in the public interest.

Revealingly, Starmer foresees the "potential for a number of protests over the coming years that may be quite large". Is there not a risk that the grounds he lists for increasing the likelihood of prosecution will reduce the chances of such protests materialising? For there is no hiding the fact that these guidelines are to serve as warnings to those wishing to participate in demonstrations in the future. "Significant disruption" being caused to the "public and businesses" is now to enhance the likelihood of prosecution. Is it possible to imagine a successful demonstration that does not cause considerable levels of disruption? Not only is the closing of major roads, bridges and shopping streets inevitably disruptive, but this guideline seems to be designed to deter protests against corporate greed of the kind seen at Fortnum & Mason last year.

If a placard can be considered a weapon, and a scarf evidence of intent to cause "trouble or disorder", if protesters' organising on social network sites are to be monitored by authorities, and protesters filmed by police whether they are engaging in illicit activity or not, is there not a danger that protesters will be dissuaded from exercising their right to public protest? Probably not, because we're made of thicker skin than that, but we could be forgiven for concluding that these guidelines are part of a broader effort to suppress dissent. What is clear is that the right to protest is in need of defending more than ever before.

• This article was amended at 13:29 on 7 March 2012. It originally stated that the student protesters amid whom police parked a van in November 2010 "were cold, thirsty and tired after hours in a 'kettle'". This is incorrect and has now been removed