A woman has described her shock after being found guilty of a public order offence for telling David Cameron he had "blood on his hands".

Bethan Tichborne, 28, said initially she assumed her court summons was a bureaucratic error after she was arrested for protesting against cuts to disability benefits. But she was told by a district judge that her comments must have hugely insulted the prime minister.

Cameron was switching on the Christmas lights in his Witney constituency in Oxfordshire, in December, when Tichborne tried to climb a barrier with her homemade placard. The one-woman protest, she claims, was against cuts leading to the deaths of people with disabilities. She believes her conviction at Oxford magistrates court last week was politically motivated.

District Judge Tim Pattinson told her: "It is difficult to think of a clearer example of disorderly behaviour than to climb or attempt to climb a barrier at a highly security-sensitive public occasion."

Judge Pattinson praised Tichborne's previous good character but said her comments that Cameron "had blood on his hands" could "hardly be more insulting to anyone, whether a politician or not".

The 28-year-old was convicted of using threatening words or behaviour to cause harassment, alarm or distress.

Tichborne, a teaching assistant who works with disabled children, said that she now intends to appeal against the conviction. "My faith in the court system is a bit shaky but on the other hand I feel I've got to hold them to account."

She added: "I am really quite shocked and quite sad … I had a placard that said Cameron has got blood on his hands and shouted disabled people are dying because of Cameron's policies.

"The judge said stuff in his verdict that made it very clear it was political, like he couldn't think of anything more insulting or offensive."

She also alleged police beat her up moments after she tried to scale the barrier separating Cameron from the crowd.

"Two lots of Cameron's undercover police grabbed me but then when I was turned over to Thames Valley police they just put me face down on the ground and beat me up quite badly. I was really scared, it felt like there were at least three or four, someone kneeling on my back, someone stamping on my knees.

"My nose was bleeding, my glasses had come off, I had no idea what was going on. I got really scared when one of them who was on my back moved up and put their knee with quite a lot of weight on the back of my neck. I started thinking: 'Oh God I can see the headlines'. I thought they were going to do real damage."

The court hearing was told that a children's choir standing at the side of the stage, waiting to perform, were terrified and that their shocked teacher heard Tichborne swear before trying to climb over the waist-high barrier.

A police officer told the court that Tichborne's demeanour was "angry, fixated, aggressive and very focused on her intentions".

Tichborne added: "I really genuinely thought it was some kind of a bureaucratic mistake that it had come to court. I really really didn't expect to be found guilty, two-thirds of the public gallery walked out in protest as the verdict was being read out."

She had wanted to read out a version of "Callum's List" which documents the names of those who have allegedly died, through suicide or natural causes, as a result of losing their benefits.

Pattinson ordered her to pay a £225 fine, a victim surcharge of £22 and to make a contribution of £500 towards court costs, which supporters of Tichborne have already raised.