The Crown Prosecution Service has dropped charges of aggravated trespass against 109 UK Uncut activists who occupied Fortnum & Mason in March, saying they are no longer in the public interest.

The Guardian can also reveal that the senior officer at the heart of policing the demonstration deceived protesters into a mass arrest.

More than 150 activists were arrested on the evening of 26 March despite holding what chief inspector Claire Clark described at the time as a non-violent and "sensible" demonstration.

As they left the store, the activists were handcuffed and transported to police stations across London, where they were held in cells for up to 24 hours.

Some protesters had their clothes and personal items taken from them for evidence purposes. One UK Uncut activist described it as "one of the worst nights in my life".

Five minors, the youngest aged 15, were among those held overnight in police cells. Their cases were dropped by the CPS almost three weeks ago. The cases of 30 activists are still continuing through the courts and will be heard from November.

Lawyers acting for the remaining defendants said the CPS decision to continue with the prosecutions "makes no sense". In court a fortnight ago the 30 were described as organisers of the protest. Thirteen have already denied the charges against them, and the other 17 are expected to do likewise during a hearing on 25 July.

Following the mass arrest, activists claimed they had been "tricked", as they said Clark had assured them that after leaving the store they would be directed to the nearest tube stations and free to go home. Their claims were backed by video footage obtained by the Guardian, which shows Clark denying that anyone would be kettled or detained after leaving the store. It was not known at the time whether Clark was aware that the activists would in fact be arrested.

The Guardian can now reveal that the police admit deception in the lead-up to the mass arrest. Clark has confirmed she was told by her fellow commanding officers that everyone inside the building would be arrested. However, 10 minutes later at 5.50pm, Clark has admitted, she gave assurances to demonstrators that they would be allowed to leave unhindered.

On the video, which timestamps confirm was shot after 5.50pm, Clark can be heard telling a member of the crowd that the police were "getting ready to let you go". In response to a question about whether there was a "kettle" outside, she replies: "No, we are getting ready to let you go."

A letter to the CPS from the defence solicitors Bindmans on 17 June criticised Clark's promises to the crowd. "It clearly brings the administration of justice into disrepute if an individual is not able to rely on the clear assurances of the police – and in this case a very high ranking officer – engaged in the policing of a peaceful protest," it read. "A decision by the CPS to continue the prosecution in these cases is an abuse of the process of the court."

Clark has defended her actions, saying she feared a breach of the peace if she told the otherwise peaceful activists inside the store that they would all be arrested.

Also that evening, so-called "black bloc" protesters broke into banks and damaged the front of the Ritz hotel. Fortnum's suffered some damage to the front of its store from those gathered outside.

The Metropolitan police said it was unable to comment on questions from the Guardian about Clark's actions. "As there are a number of outstanding trials, it would be inappropriate to comment at this time," a spokesperson said.

The CPS had signalled a fortnight ago that it would drop "a significant" number of charges. In a statement on Monday Alison Saunders, chief crown prosecutor of CPS London, said: "I have decided that the cases against 109 individuals who were charged after protesting in Fortnum & Mason on 26 March should be discontinued. However, we will continue to pursue the other 30 cases through the judicial process.

"After careful and necessary investigative work by the police to provide the fullest evidential picture, we have reviewed the evidence now available and considered representations made by the defence teams.

"I have concluded that there is sufficient evidence to provide a realistic prospect of conviction for the offence of aggravated trespass against all the defendants. However, in discontinuing these cases I have considered whether a prosecution is necessary in the public interest. I have decided that it is not, taking into account the following factors from the code for crown prosecutors: that this was a single incident; that they have not been involved in similar offences previously and that they played only a minor role in the offending behaviour; that if these defendants had been convicted the court would be likely to impose only a nominal penalty.

"The police investigation was able to identify whether these factors were present for each defendant. It was only after such an investigation that these factors could be applied," she added.

Mike Schwarz, of Bindmans, said: "It makes no sense to continue the prosecution of some of them while they are discounting so many of the other cases. The nature of civil disobedience doesn't accommodate that discrimination."