A crowd of 500 had gathered outside the site of one of the Government's farm-scale trials to voice their opposition.
Campaigners said the atmosphere resembled a picnic with families enjoying the summer sun. Many wore fancy dress for the day, with some clad in butterfly or corn-on-the-cob outfits. Hand in hand they circled the 25- acre field to make their point.
However, a group of about 100 broke away from the main crowd, ran on to the site and began ripping up the oilseed rape and rolling on top of it.
"People are digging up the fields," said a spokesman for Greenpeace. "They have taken it upon themselves to decontaminate the fields of GM crops."
A spokesman for Genetic Engineering Network, a campaign group, said: "This whole experiment has been completely invalidated by the destruction." Eighty police tried to stop the damage at Model Farm at Stoke Talmage near Watlington, Oxfordshire. One policeman suffered minor injuries and three protesters were arrested for criminal damage.
"As a result of the actions of a section of the march we had to make arrests," said a spokesman for Thames Valley Police. "We will also be instigating an inquiry against others who were involved."
Model Farm, one of six farm-scale GM trials in the UK growing rape and fodder maize over four years, was also the target of a protest in July last year and one of its farmhouses was occupied by demonstrators last month.
The Government is to review and could increase the amount of money it puts into the organic food industry after the backlash against genetically modified foods and other safety scares.