Fourteen animal rights campaigners walked free from court today after a judge condemned the police for unlawfully stopping their protest.

The protesters were all cleared of refusing police orders to break up the demonstration against Oxford University's animal research laboratory outside the Sheldonian Theatre at the Encaenia ceremony last July.

A tape-recording emerged during the trial of unguarded comments by officers, calling the campaigners from the Speak animal rights group "c****", saying they wanted to "prosecute the s*** out of them" and claiming the university was powerful.

The comments, caught on a dictaphone accidentally switched on inside an officer's pocket, led to Speak accusing the force and university of colluding to stifle their right to protest.

During last summer's protest, demonstrators refused to move from outside the Sheldonian Theatre, where the Encaenia ceremony was being held, under police powers known as section 14s, and instead sat down.

At Bicester Magistrates' Court, District Judge Deborah Wright cleared all 14 defendants of offences under section 14 of the Public Order Act.

But she found 70-year-old Pauline Broughton guilty of obstructing a police officer and Fran Cornwell guilty of assaulting a police officer - and gave them both absolute discharges.

Ms Wright said: "I find the (section 14) conditions were imposed unlawfully.

"Whoever was responsible for making the decision that this prosecution should proceed in light of the tape may well have made a serious error of judgment."

She added: "Although the (taped) conversations were made away from the public, all the officers were on duty. Pc (Colin) Travi accepted he had referred to them (the protesters) as c****."

He was also recorded saying "the problem is the protesters do not realise how powerful the university is - it's a sleeping giant".

Pc Andy Bignall was recorded saying: "We knew we were going to take bodies today."

Supt Chris Shead said: "Now we have to prosecute the s*** out of them."

And Ms Wright added Chief Insp Chris Sharp admitted the measures they imposed were "draconian".

Ms Wright said the protesters had been put under a "metaphoric microscope" by police.

Outside court, Speak spokesman Mel Broughton said: "I am extremely angry and disturbed about what has been revealed of the attitude of Thames Valley Police.

"I think anyone in the country who believes you have a right to protest and a right to free speech should be very, very concerned."

Referring to police comments about Oxford University, he added: "These two extremely powerful institutions are prepared to get into bed with each other to frustrate lawful protest."

Thames Valley Police Deputy Chief Constable Alex Marshall said: "I have listened to what the judge has had to say and I take her comments very seriously and will now review this case to see whether there are any disciplinary matters that need to be dealt with.

"There are comments on the tape that I find very regrettable and I find some of the comments unprofessional. I will take careful note of what the judge has said and see if there are any matters which arise from it."

Referring to the comments about Oxford University, he added: "Those are the comments of a constable after an operation. There's no collusion."

A spokesman for Oxford University said: "While we are in regular dialogue with the police, operational matters are entirely within their jurisdiction, and are not a matter for the university."