The picnic is being held in the grounds of the National Maritime Museum in Greenwich, London. The feast of chemical-free food included a giant fruit salad prepared by the Savoy Hotel.

Among the protesters were Tom Parker Bowles, son of Camilla, and Zac Goldsmith, son of the late Sir James Goldsmith, accompanied by his wife.

Mr Parker Bowles said he was "completely against GM food", and signed one of thousands of postcards that will be sent to Prime Minister Tony Blair urging him to ban GM foods.

Zac Goldsmith and his wife Sherherazade support the cause

Zac Goldsmith said: "It's very encouraging that so many people have turned up here today."

People were invited to bring their own organic picnic to show their opposition to the introduction of GM ingredients in food. It has been organised by a coalition of anti-GM organisations, including including Greenpeace, the Soil Association and The Picnic.

The organisers have also jointly challenged the government to set a target for organic agriculture in the UK which at least equals that of the rest of the European Union - 30% of agriculture by 2010 if current rates of growth continue.

At present 70% of organic food sold in the UK is imported and just 1% of agricultural land is farmed organically.

Greenpeace's campaign director John Sauven said more and more people were switching to organic foods.

Organic food is 'solution'

He said: "Organic food is taking off in a way that people could not believe a year or two ago.

"Organic food is a solution to the whole controversy over GM foods because it is not just GM free, but it also has no synthetic chemical inputs like pesticides and fertilisers.

"Animals are allowed to roam freely, they are not given antibiotics or growth promoters, so we really see organic food as solution to protecting the environment and human health."

Picnic attractions included live music, children's entertainment and free organic food tasting. It is one of several to be held during the summer.