Two women accused of sabotaging genetically-modified crops in north Wales have denied causing the damage.
Protesters in a field of GM crops in Sealand
Yvonne Davies, from Carmarthen and Rowan Tilly, from Brighton, both aged 45, deny damaging a crop of GM maize at a field in Sealand, Flintshire, during a protest in July 2001.
The defendants were among about 100 protesters at Birchenfield Farm, which was playing host to the crop trial.
Farmer John Cottle was paid £1,000 for growing the crop on his land in a trial conducted under EU directives.
The accused women are expected to claim they had a lawful excuse for their actions - both previously said they were defending the environment and it was a case of public interest.
Opening the case at Mold Crown Court, barrister Matthew Dunford said: "Make no mistake about it, whatever the wishes of the defendants may be, this case is not about whether genetically modified crops are desirable.
"GM crops are not on trial in this court today. That is not the question that
you are being asked to consider."
He said it was the prosecution's case that the women had taken the law into their own hands.
The defendants have accepted the prosecution evidence against them and the jury has been told the only issue is to determine whether the defendants had a lawful excuse for their actions.
The court heard protesters went to the farm following a meeting at Connah's Quay, and asked farmer John Cottle if he would pull up the crop, but he refused.
Mr Cottle had been taking part in government-funded trials of the crop, owned by Aventis Crop Sciences Ltd..
The crops, and some conventional maize growing alongside it, would have been disposed of after the trial had finished.
The court heard that 100 or more protesters - who entered the 10-acre field at Birchenfield Farm waving flags and banners and blowing whistles - "went wild" and started breaking down the crop.
Police in an overhead helicopter made a film of the whole protest, which the jury were told they would be shown.
Security guards were unable to deal with the protest and police were called in, making several arrests.
The court heard between one and two and a half acres of the GM crop were destroyed and about a quarter of an acre of conventional crops.
The prosecution said it was difficult to quantify how much the damage was worth.
Judith Jordan, product development manager at Avensis, said any damage to crops before harvesting had a "deleterious effect" on the experiment, which was being conducted under EU directives.
Earlier, up to 20 gathered on the court steps to show their support for the defendants.
The demonstrators represented groups ranging from Friends of the Earth and the Green Party to GM protest groups from as far afield as Hereford and Carmarthen.
Banners with anti-GM slogans were displayed by the participants.
The defence is expected to call a number of expert witnesses, including two genetic scientists from the organisation Genewatch.
Wales MEP Jill Evans and environmental journalist George Monbiot will also be called.
Lawyers are expected to argue that the actions of the women were in the public interest.