Activists wearing masks of Prime Minister David Cameron have handed over a petition to Downing Street to protest against the cloning of cattle for food.
The 35 men gathered in Trafalgar Square before marching on to Downing Street.
The open letter to Mr Cameron from Compassion in World Farming (CIWF) has received more than 7,000 signatures.
The group said cloning is cruel and called for meat and dairy from cloned cattle and their offspring not to be allowed to enter the food chain.
'Unauthorised and unwanted'
The charity's protest comes after it emerged that a British farmer had mixed milk from the offspring of a cloned cow into the farm's daily production, without marking it.
Following the claims an inquiry was launched by the Food Standards Agency. The FSA also found that meat from the offspring of a cloned cow entered had entered the food chain in 2009.
The demonstrators were carrying banners which said "Cloning = Cruelty".
CIWF has also called for an EU-wide ban on the use of cloned animals for food production, following an "overwhelming vote" on the issue in the European Parliament in July.
Its chief executive, Philip Lymbery, said: "We're here to send a clear signal to David Cameron's government that cloning equals animal cruelty.
"Cloning has slipped into our food chain unnoticed, unlabelled, unauthorised and unwanted.
"It is time to put that right and to ban the marketing of cloned animals and their offspring for food."