Protesters planning action over a genetically modified wheat trial in Hertfordshire have given up their plan of staging a demo near the crop.
Police say around 200 protesters from a group called Take The Flour Back gathered near Rothamsted Research in Harpenden.
Extra police were drafted in to stop them going on to the land, amid fears the demonstration could result in damage to crops and buildings.
The plan was to walk along footpaths to a site where scientists are growing a new strain of experimental wheat.
The research aims to see whether wheat engineered to deter aphids can reduce the need for pesticides, but the protesters say the experiment poses a threat to the environment and want to see it stopped.
Take The Flour Back's website bills the demonstration as a "nice day out in the country" to "share picnics, fun, and flour frolics".
However, scientists feared an attempt would be made to wreck the experiment.
In their letter, sent earlier this month they wrote: "Dear Take The Flour Back, we have learned that you are planning to attack our research test site on 27th May.
"We know we cannot stop you from taking the action you plan, nor would we wish to see force used against you. Therefore we can only appeal to your consciences, and ask you to reconsider before it is too late, and before years of work to which we have devoted our lives are destroyed forever."
The scientists argue the moral case for their research, stressing that they agree agriculture should seek to work "with nature rather than against it". In a plea for common sense, they say: "What you are planning to do is reminiscent of clearing books from a library because you wish to stop other people finding out what they contain."
At a recent press conference in London, the researchers told how they had inserted a peppermint gene into wheat so it released an aphid-deterring pheromone.
The smell is recognised by the aphids as an "air raid warning" of an attack by a predatory parasitic wasp.
Scientists hope it will keep aphids away and attract wasps to further control the pests.
Eight plots of land, each six metres square, have been planted with the GM wheat.
A spokeswoman for Take The Flour Back, who called herself Helen, said members may take action to remove the crop if the experiment is not halted.
"We're asking the scientists to do the responsible thing and stop this trial," she said. "If they don't, we're going to have to do the responsible thing ourselves.
"We are a broad coalition of people - bakers, farmers, school workers - and it will be up to individuals to decide what they want to do.
"We're not taking this lightly. We really don't want to be pushed into this action. We're not anti-science. It's just about doing things responsibly.
"This wheat uses a synthetic gene that's most like a gene from a cow. It's quite weird technology that hasn't been properly tested and should not be tested outside.
"It's a threat to the environment, and also we feel it's about making money, not about feeding the world and helping UK farmers. There are other solutions, like organic farming."
Asked if the group would call off the protest in response to the scientists' plea, she said: "No, we will not."
Hertfordshire Police said they "fully recognise the right of people to demonstrate lawfully and peacefully on public land and will facilitate demonstrations of this nature in Hertfordshire. Officers will be present to police the day peacefully and ensure that people are able to protest peacefully and we would welcome further contact from the organisers in order to facilitate this. However, as the police we always have a duty to uphold the law and will respond appropriately should and unlawful demonstrations or criminal acts occur.”