Farmers and consumers joined together in a protest march on Monday against the introduction of genetically modified food and crops in the UK.
Many protesters wore costumes to highlight their case
The rally in London was timed to raise awareness before the government announces the results of field trials on GM farming on Thursday.
The tests compare use of weed killers on GM and non-GM maize production, and are expected to show that GM farming is more eco-friendly.
On Sunday, former environment minister Michael Meacher launched a scathing attack on the trials, claiming a recent EU weed killer ban rendered the results invalid.
But the government has denied the ban meant the tests were flawed.
Mr Meacher was expected to join the rally alongside Indian ecologist, activist and author Vandana Shiva.
Campaigners said about 20 anti-GM "pilgrims" who had been travelling across the UK by foot, bike or tractor to raise awareness also gathered at the rally.
Some had travelled more than 600 miles.
Organic farm worker Jonny Barton was due to arrive in London having cycled from Inverness towing a coffin.
And Writer Martin Haggerty had set off from North Yorkshire on foot in September in time for the event which was organised by Friends of the Earth, GM-Free Cymru, Genetic Engineering Network and Five-Year Freeze.
Blow to trials
The parade took protesters from the National Farmers Union headquarters, to Downing Street and the Department of Environment, Food and Rural Affairs, where they handed in messages opposing GM.
Friends of the Earth argues the GM tests "compared two unsustainable ways of growing maize" and failed to take into account other issues such as cross-pollination with neighbouring crops and soil erosion.
Mr Meacher - who set up the tests - said an EU ban on a weed killer used in the tests called atrazine meant the three-year tests must restart.
"We need to try the trials again with a different herbicide to see what the comparison is between that and the GM one.
"I cannot see that the government could logically, consistently, or morally go ahead when the comparison is exposed to everybody as not being a valid or a real one."
But a government spokesman said herbicides other than atrazine, which is suspected of causing cancer, had also been used in the trials.
The findings of the trials are likely to have a huge influence on the final decision on commercial production of GM crops in the UK.
It was reported that this week's results would show herbicides used with two of the three GM crops tested - oilseed rape and sugar beet - were more damaging to insects and plants than normal weed killers.