The protest, organised by the Genetic Engineering Network (Gen), chose Model Farm near Watlington - one of six government-run farm scale-trials in the UK - for the rally.

As demonstrators left one field, they reportedly split into two groups and stampeded into the trial site, rolling over plants and ripping up handfuls of the crop.

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

A police spokesman said: "Due to the nature of the terrain of the field it was impossible to defend all access points from invasion and although we were assured that this was to be a peaceful protest it has turned into an unpeaceful one.

"We did everything possible to facilitate a peaceful protest and as a result of the actions of a section of the protesters we will now be making arrests for criminal damage.

'Decontamination process'

"We will also be instigating an inquiry against those involved to make sure that they are tracked down and arrested."

A spokeswoman for the environmental group Greenpeace defended the actions.

"It is a non-violent protest, it's just a decontamination process," she said.

Alan Simpson MP, the author George Monbiot and Lynda Brown were among those who addressed the rally.

"There has been overwhelming interest from groups and individuals wanting to voice their opposition to these crops," a Gen spokesman said.

He said Gen intended the rally to be a peaceful protest.

Funding review

On Sunday it 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.

Organic food production in the UK is worth £400m a year

Countryside Minister Elliot Morley said the review could result in more government funding for organic food producers.

The government currently hands out £6.5m a year to farmers to help them to convert to organic methods, which usually takes about three years.

Organic food production in the UK still only accounts for 1% of all food production but is currently worth about £400m a year while 75% of organic food sold in the UK is imported.

Anti-GM and environmental campaigners are pressing the government to hit a target of 10% of all food being organic by 2005.