They were, say prosecutors, the ringleaders of one of the most high-profile protest acts of recent years – the occupation in March of the luxury Fortnum & Mason store in Mayfair during an anti-cuts demonstration. But supporters of the 30 activists whose trial starts this week say their treatment is irrational and vindictive, and that charges of aggravated trespass should be lifted, as they were for more than 100 of their fellow UK Uncut activists last week.

Lawyers have written to the Crown Prosecution Service ahead of hearings, claiming that the continued targeting of selected activists could amount to a misuse of the law to undermine peaceful protest.

Prosecutors remain intent on taking 30 "ringleaders" to trial. Critics claim police are pushing ahead with the case in order to "break" UK Uncut and put an end to the growing trend for "direct action" by demonstrators.

Protesters in Fortnum & Mason were told by police that they would not be arrested if they left the store peacefully, but it emerged last week that their subsequent detention had been planned.

A letter sent by law firm Bindmans to Alison Saunders, the chief Crown prosecutor, and to Keir Starmer, the director of public prosecutions, on Friday claims that the CPS and police have attempted to distort the intentions of protesters and "portray the core of this action as disorderly and violent".

Mike Schwarz, a solicitor for Bindmans, also said the Metropolitan police had failed to answer a query in April over whether undercover officers had been involved in the Fortnum & Mason protest against tax evasion. The lawyer has requested that any information relating to undercover officers "in the infiltrations of the demonstrations or prosecutions" of any of those arrested at Fortnum & Mason be disclosed.

Last week the convictions of 20 environmental protesters for attempting to shut down a power station were quashed after judges ruled that crucial evidence recorded by a police spy, Mark Kennedy, had been withheld.

Lawyers claim they have received no evidence that any of the 30 still facing prosecution were in any sense "organisers" of the UK Uncut occupation of the luxury department store on 26 March, which coincided with the TUC anti-cuts march in London. The group says it has no central leadership.

The Observer has learned the details of the cases assembled by prosecutors against the activists, including the sometimes bizarre grounds selected for identifying the supposed organisers of the protest. A 24-year-old man is facing trial on the basis that "he entered the store with a placard". Lawyers from Hodge Jones & Allen, who are also representing the protesters, say no information has been provided relating to when he acquired the placard or even what it stated.

In a separate letter to the CPS, solicitor Raj Chada, head of the protest law team at Hodge Jones & Allen, said: "We understand that the banner is in relation to 'anti cuts' generally, rather than specifically UK Uncut, and is similar in type to others carried by the 500,000 who attended the March for the Alternative."

Chada said it was bizarre that protesters would escape prosecution simply because they were not holding a placard. "One individual will face no criminal prosecution; the other faces a prosecution that can subject the defendant to a potential prison sentence of three months. This is decision making that is arbitrary, irrational and unreasonable."

Others charged include a 23-year-old woman, who "unfurled a notice on the stairwell" of the store. Another protester is facing prosecution "due to her placing leaflets on displays". A third is referred to in the documents as "carrying an umbrella into the store". One 22-year-old participant is accused of carrying UK Uncut cordon tape and more than 50 printed signs stating: "Big society, revenue and customs: if they won't chase them, we will."

Prosecutors, according to lawyers, also targeted people caught carrying 20 or more UK Uncut leaflets. "The Crown has treated this as a 'tick box' exercise rather than a proper evidential evaluation of organisational involvement," said Chada.

Some activists have been targeted on the basis of previous convictions of aggravated trespass. Bindmans' letter points out that some have convictions for aggravated trespass during protests in 2009 at Ratcliffe power station, the case that became notorious for the role of Kennedy. Schwarz said the singling out of UK Uncut members over previous protests would seem "to an outside observer irrational and vindictive". The CPS, however, maintained yesterday that there is "sufficient evidence against the 30" for them to face trial.

A spokesman said: "We have identified 30 defendants whose actions, the evidence shows, were more culpable than the others, such as where there is evidence that they organised the action, or where defendants have relevant previous convictions."

Seventeen of the protesters will appear at a City of Westminster magistrates court tomorrow. Thirteen have already denied the charges against them, and the rest are expected to follow suit.

About 150 activists were arrested in Fortnum & Mason despite holding what Chief Inspector Claire Clark described as a non-violent and "sensible" demonstration. As they left the store, the activists were handcuffed and taken to London police stations, where they were held in cells for up to 24 hours.

Three days after the arrests, Lynne Owens, assistant commissioner of the Met, told the home affairs select committee that police arrested so many people that day because it gave them "important intelligence opportunities". The case has proved embarrassing for the Met, with claims that the senior officer at the heart of policing the demonstration deceived protesters into a mass arrest. Video footage shows Clark denying that anyone would be detained after leaving Fortnum & Mason.

UK Uncut sources claim that the Met was so desperate for intelligence on the relatively new protest group that it undertook mass arrests to learn more about the command structure of the organisation. The 30 alleged ringleaders include teachers, charity workers, town planners and religious studies tutors who lawyers say have had their lives "severely disrupted" since their arrest four months ago.

The trials, if they go ahead, are likely to be held in November.