More than 200 human rights activists and environmentalists marched to the gates of the Defence Systems Equipment International 99 show in Chertsey, Surrey.
Chanting slogans and waving banners the protesters crowded around the main entrance. Scuffles broke out as some broke through police lines and lay down in front of cars.
The demonstrators were read their rights by police before being dragged off to rejoin fellow protesters. One man was later arrested from under a minibus where he had managed to handcuff and chain himself, police said.
The demonstrators objected to the show being held on Ministry of Defence land and being opened by Lord Robertson of Port Ellen, the Defence Secretary.
One man set up a shrine with Tibetan peace flags blessed by the Dali Lama and said he would be holding a 24-hour-a-day peace vigil. He said: "We're holding the prayer vigil for the victims of war and weapons of destruction. We hope people will join us in prayer throughout." More than 20,000 delegates are expected to visit the exhibition which takes place over four days in Surrey and London's Docklands.
Police later closed the road outside the main entrance to the exhibition and delegates had to walk past demonstrators who chanted: "Shame on you".
Jeremy Corbyn, the Labour MP for Islington North, spoke to the crowd of his experiences as an observer at the elections in East Timor and he admonished Robin Cook, the Foreign Secretary, for continuing to sell arms to oppressive regimes.
Mr Corbyn added: "Visitors are coming here to buy weapons that will be used to kill people at some time somewhere in the world.
"There is no point in condemning human rights abuses around the world if we are selling weapons which the death squads are using."
He said British-built Hawk aircraft had been used by the Indonesian military in East Timor and that sensitive communications equipment built in the UK was also being used.
Rachel Harford, a spokeswoman for Campaign Against the Arms Trade, said: "We are not prepared to allow Tony Blair's Government to host an exhibition in our name and with our money where delegations from human rights abusing countries will doubtless be invited to buy arms."
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