Hundreds of anti-GM protesters gathered near a scientific research centre on Sunday where a genetically modified wheat crop is being grown.
Members of the anti-GM movement Take The Flour Back had planned to rip up the crops which they fear could "contaminate" nearby fields, prompting St Albans City and District to apply to the Home Secretary for greater police powers.
The order they were granted prevents anyone going on the land belonging to Rothamsted Research until midnight tonight.
After the heavy police presence around the centre and a nearby park, where activists were gathering before walking to Rothamsted, none of the planned "decontamination" was able to take place.
However two men were arrested on suspicion of public order offences during the demonstration, Hertfordshire Police said.
A force spokeswoman said: "The men were arrested from within the confines of the trespass banning areas and they are being taken to a police station."
They were arrested under Section 14c of the Public Order Act, which stops people from holding "an assembly" on a prohibited area of land.
Instead, protesters linked arms to walk to the site before reassembling in the park where they spent time "chatting, listening to music, sharing non-GM food and swapping plants and seeds", a spokeswoman for the group said.
Take The Flour Back said the demo had been a success as it had reopened the debate about GM crops. Group member Nicola Gomez said: "People came from across the country. It went peacefully.
"I feel today has been a way to raise awareness again in the public domain about GM crops which are being tried out in the UK, despite there being public opposition to them."
Speeches were made from organisers and other groups who came out to support the cause, including the African Biodiversity Network.
Their coordinator Gathuru Mburu told those assembled: "Experimenting with staple crops is a serious threat to food security. Our resilience comes from diversity, not the monocultures of GM."
A Rothamsted Research spokesman said: "We are pleased that the protest in Rothamsted Park this afternoon has been peaceful and that both supporters and opponents had the chance to make their voices heard.
"We have worked closely with the Hertfordshire Constabulary and are very grateful for their support to protect any illegal damage to our experiments and facilitating this peaceful protest.
"The police and security response was necessary to prevent the destruction of our work and, with the constant threats to 'decontaminate' our experiment as well as the vandalism last weekend, it was important to ensure a safe afternoon for our staff, visitors and protesters alike.
"We hope we can now proceed with this BBSRC-funded project without any further threats to find some answers to the question of reducing the use of harmful insecticides in wheat crops, benefiting future generations and the environment."
Professor Maurice Moloney, director of Rothamsted Research, said that the protesters' decontamination plans would have been unnecessary anyway, as they insisted the chance of any cross-pollination was minuscule.
He said: "We have no idea who is advising them scientifically, because it is absolutely incorrect.
"Wheat is a self-pollinating plant so there is virtually no chance of any cross-pollination with local wheat.
"The way we have grown the wheat is desynchronised with local wheat as well, meaning it flowers at different times. It is the equivalent risk of worrying that a tornado would hit you."
The experimental crop of GM wheat was planted in April and is due to grow until September. It has been developed with genes from the mint plant.
Prof Moloney explained: "As a result, the wheat produces a volatile chemical which the aphids don't like and it makes them go elsewhere.
"The purpose of it is to see if we can come up with a strategy that would avoid the use of pesticides in wheat crops, as they kill other creatures like bumblebees."
He welcomed a debate with the protesters, adding: "There should be an exchange of views and it should be done in a civilised manner."
A message on the Take The Flour Back website reads: "This open air trial poses a real, serious and imminent contamination threat to the local environment and the UK wheat industry... This crop is a threat to our thriving wheat industry and our food security. This action is for anyone who feels able to help take it out, and all those who wish to support them."
They advised people attending to bring their own "biohazard protection and dust mask" if possible.
The Government's chief scientific adviser, Professor John Beddington, told BBC Radio 4's World This Weekend: "No one would claim that GM is a way of solving the food security issue, but it is one of those tools that actually has that potential.
"The attraction of GM is that when it happens it may be able to solve some problems you can't solve in other ways."
Prof Beddington said ongoing research was needed to explore potential benefits from GM wheat, adding: "It properly has got to be done in very carefully controlled conditions that minimise as far as possible any problems, and that's exactly what's happening."
Hertfordshire Police has said earlier that they were said they were monitoring the situation, estimating there were 200 protesters taking part.
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