Police use trespass order and mounted officers to halt hundreds of activists at entrance to land owned by Rothamsted Research

Police kept hundreds of protesters at bay as they attempted to destroy a field where genetically modified wheat is being tested in Hertfordshire.

Mounted officers helped bring activists to a halt in front of the entrance to land owned by the Rothamsted research institute, which is testing GM wheat which has been enhanced to fight aphid infestation.

The event on Sunday, which attracted hundreds of protesters including farmers, politicians and activists from the UK and abroad, had prompted the local council to obtain an order making it a criminal offence to trespass on the land.

Hertfordshire police handed out leaflets at Harpenden station warning that St Albans council had forbidden "trespassory assembly" under section 14A of the Public Order Act as anti-GM campaigners gathered in a park next to the estate.

Lucy Harrap, 36, who helped organise the Take the Flour Back event, said there was a "groundswell of support" for the action among the UK public.

Harrap said widespread use of GM would remove choice from consumers.

"If this wheat goes to commercialisation, there would then be cross-contamination and we would no longer have a choice about GM or non-GM. When that happens it is not going to be Rothamsted who are going to pick the tab up – it is going to be farmers in this country."

The large police presence and the public order enforcement notice seemed to have deterred campaigners from ripping up crops, one of the stated aims on the event's website.

During a series of speeches legal observers told the group that they were very likely to be arrested and face charges for any attempt to gain access to the site.

A representative from Citizens Concerned Against GM urged the crowd not to rip up crops and instead "process around the site" as activists had already "won the moral high ground" in the public sphere.

Activists decided to link arms, march towards police lines and remain seated in front of officers, some on horses.

Protesters said the legal and police response indicated a crackdown on such protests. Harrap said activists were facing a different situation from that seen 10 years ago. "The corporations behind biotechnology were totally unprepared for what happened 10 years ago. They were unprepared for that wholesale rejection by the UK public. They are not unprepared now and what we are seeing is the result of that. We are going to see the state absolutely protecting sites like Rothamsted doing that research and this is the thin edge of the wedge."

Before the event the scientists had attempted to persuade the protesters to abandon their action, arguing in a video uploaded to YouTube that they were publicly funded researchers.

Sir Paul Nurse, the president of the Royal Society, said on Friday: "Scientific discoveries can be unsettling and their application for societal benefit complicated to implement. That is why we must have informed public debate on these issues, free from hype and fear. We need to do scientific experiments to find out if genetically modified crops are safe and if they deliver genuine public benefit. If they fail on either score, they should be put to one side. That is why the trial at Rothamsted should be allowed to go ahead."

Professor Douglas Kell, the chief executive of the Biotechnology and Biological Sciences Research Council (BBSRC), the government body funding the trial, said: "Now that the protest at Rothamsted has ended peacefully I hope that the BBSRC-funded scientists can be allowed to complete their project without the ongoing threat that their work will be destroyed.

"As scientists, we do not claim to have all the answers. However, our scientific community must be able to conduct regulated and approved trials and experiments without the threat of vandalism hanging over them."

Among the protesters were several groups from abroad. Franciska Soler, a member of the French group the Faucheurs Volontaires d'OGM – Volunteer Reapers of Genetically Modified Organisms – which had 18 members at the protest, said: "We think that European resistance against GMOs is very important … we are probably the only continent that refuses GMOs and are still resisting."

Soler, who said the organisation had nearly 7,000 members in France of all ages, pledged to resisting genetically modified food. "I think research has to be reorientated, not just around a debate around science but something that is a partnership with civil society and to follow what society wants."

The prominent Green party member Jenny Jones, who was at the event, said she was relieved the day had not ended in violence. "We're gathered here because we are all so frustrated about this trial. It's been a great day out with some lovely people. We've listened to some good music with some good speeches."