Hundreds of anti-GM protesters are demonstrating close to the site of an experimental crop of modified wheat in a Hertfordshire field.
Rothamsted Research hopes the modified crops being tested in Harpenden can deter aphids, a major wheat pest.
Members of the anti-GM movement Take The Flour Back said they 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.
There is a heavy police presence around the centre and a nearby park, where activists were gathering before walking to Rothamsted.
Earlier this month, scientists at Rothamsted Research sent an open letter to protesters after plans were announced to visit the site by Take The Flour Back.
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.
However, protesters could not access the land following the order granted by the Home Secretary.
Please wait while this video loads. If it doesn't load after a few seconds you may need to have Adobe Flash installed.
Dr Gia Aradottir, project scientist at Rothamsted Research, 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 in their support to protect the illegal damage to our experiments and ensuring and facilitating this peaceful protest."