A sixth person was also taken into custody for alleged violent disorder during a mass protest at a 25-acre field of genetically engineered oil seed rape in Watlington, Oxfordshire.

Demonstrators reportedly split into two groups and stampeded onto the trial site, rolling over plants and ripping up handfuls of the genetically engineered oil seed rape.

Chief Superintendent Mick Page added: "We had agreed a route for the protesters to follow enabling them to get close to the field and make their point in a lawful manner.

One police office sustained a minor knee injury during the protest

"We did of course make contingency plans for some form of additional protest, despite assurances from the organisers that they have no wish to break the law.

"Contrary to previous assurances, all 500 demonstrators entered the field en masse and a significant number deliberately began to damage the crops.

"Because of the layout of the terrain, it would have been impossible to cover every possible access point which could have been used by protesters to enter a field which is the size of 28 football pitches."

Field flattened

Protesters at the site destroyed the field that is half genetically engineered and half normal crop, said a spokesman for campaign group the Genetic Engineering Network.

The BBC's Emma Massey: "Police later intervened"

"The half of the field that is genetically engineered has been destroyed. It has been completely flattened.

"This whole experiment here has been completely invalidated by the destruction that's been carried out by activists today," he added.

"It is the only option after the breakdown of the political debate.

"Campaigners have to take it into their own hands and stop the crop themselves. There has been no violence at all. It's been completely peaceful," he added.

The destruction of the crop came after a mass rally earlier on Sunday at Manor Farm, attended by more than 500 people.

Following speeches against GM engineering, the crowd - wearing white decontamination suits - moved to the field growing the GM oil seed rape and began to dig up and trample on the crops.

On Sunday it also emerged the government was to review the amount of money it puts into the organic food industry after the backlash against genetically modified foods, and other safety scares.