Most of the people arrested during a summer of demonstrations against fracking in the village of Balcombe have been acquitted, leading to accusations that police tactics in a £4m operation criminalised peaceful protest.

The last of the criminal trials resulting from 126 arrests made by Sussex police during days of action outside the Cuadrilla site last summer finished this month. Of 114 charges, relating to 90 individuals, only 29 resulted in convictions, according to freedom of information responses from the Crown Prosecution Service (CPS) and the police.

Sussex police are accused of using mass arrests, draconian bail conditions and section 14 notices under the Public Order Act 1986 to criminalise peaceful protest at the site in Balcombe, where the energy firm Cuadrilla conducted exploratory drilling.

Lydia Dagostino, the solicitor who acted for most of the defendants, claimed many believed the policing operation was used as a blueprint for the future policing of anti-fracking protests. She said: "Who knows if there was some kind of directive given about how they tackled this – 'this is the energy industry do what you want to do'. It was almost like they [the police] said 'we are going to arrest people and justify it later'.

"What they did criminalised protest. They used the section 14 orders and bail conditions, which were imposed on everyone and which stopped them from going within miles of the site, to stop them from protesting. It was like an injunction by the back door. If you turn up – new to protest – and you think you are going to be sitting down singing the anti-fracking anthem and then see people being arrested and handcuffed, it is quite shocking and frightening and puts you off being there."

The last of the trials involved Simon Welsh, a poet from Balcombe, who was arrested for failing to comply with a section 14 order on 9 September as he led the crowd outside the Cuadrilla site in a song.

Welsh, 35, was acquitted at Eastbourne magistrates court this month. He is one of 15 people, including the Green MP Caroline Lucas, acquitted at magistrates court of breaching such orders at the Cuadrilla site. A related section 14 trial due in September has been dropped by the CPS.

Welsh said: "I was leading people in song. I was holding a microphone of a loud hailer, and I was belting out lyrics when I was arrested.

"We are talking about people standing for something they believe in in a way that, to me, seems to make the rules no longer applicable. Police action becomes questionable; the law as it stands becomes questionable."

In another case, known among demonstrators as the log trial, 11 individuals arrested for obstructing the highway on 26 July by sitting on a log outside the Cuadrilla gate were also found not guilty.

District judges have challenged the validity of the section 14 notices issued by the police during the protests. The order effectively throws a protest-free exclusion zone around a site but can be issued only if a senior officer believes there will be serious damage to property, serious disruption to the life of the community and serious public disorder.

In one ruling, the district judge at Brighton magistrates court said the notice was invalid.

"The words 'serious' in section 14 (1) (a) are inserted for a reason. If a police officer merely believed that public disorder, damage or disruption (which were not serious) were anticipated, there would be no ground for approving a notice.

"I have very real concerns that there were grounds for reasonable belief that a future assembly may result in the serious consequences envisaged by section 14 … I have such concerns about the notice … that I find it to be invalid."

Ch Supt Paul Morrison, of Sussex police, defended his force's response: "Protests are an important safety valve for strongly held views and the right to protest in public is a synthesis of iconic freedoms: free assembly and free speech," he said.

"We are looking at the judgments and the implications for policing future events of this nature but I am satisfied that public order policing in Sussex is conducted professionally and fairly."

Cuadrilla began drilling activity in Balcombe last summer. More protests are likely in the area after the firm was granted permission last month to carry out further tests before sealing off the well.

Cuadrilla has said it will not frack for oil at the site, but wants to check oil supply suitability using flow-testing.