Campaigners against Europe's largest international arms show say half of the countries invited to the event next week are not allowed to buy weapons under British law.
Attack helicopters: Among weapons on display
Up to 60 groups from the UK and Europe say they will hold protests against the event, which is expected to be attended by up to 20,000 top military and government figures from around the world.
But police warn protests may spill over into violence and anti-terrorism officers will be on stand-by as part of a huge operation.
Defending the decision to hold the fair, a spokesman for the MoD's sales organisation said: "We have strict export licence criteria and going to the exhibition in no way implies that you can buy the equipment."
And the organisers, Spearhead, said: "This is not an arms fair. If you want to come and buy 500 Kalashnikovs and 50,000 rounds of ammunition and have them delivered next Wednesday, you can't."
The four-day Defence Systems Equipment International (DSEI) show in London's Docklands, which begins on Tuesday 9 September, includes 950 exhibitors, warships on the Thames, helicopters fighter aircraft, tanks and smaller weapons.
At least one manufacturer of controversial cluster bombs is thought to be attending.
The Campaign Against Arms Trade (Caat) said an alliance of local, national and international protest groups would do their best to disrupt the event.
Gideon Burrows, author and campaigner, said: "This is the third time this arms fair has taken place in London's Docklands.
"Our view is at least half of the countries would not be allowed to buy arms under the 2002 Arms Control Act."
The legislation bans arms sales by British companies to regimes which could use the weapons for either internal oppression or external aggression, said Mr Burrows.
"But half of the companies exhibiting are not British. So if a deal takes place between Pakistan and a Chinese company then there is nothing we can do about it, even if it is in the UK.
"Fifteen years ago Saddam Hussein's henchmen were shopping for arms in Aldershot. Next week we will be selling arms to the Saddam Husseins of the future."
Fears of violence
Metropolitan Police deputy assistant commissioner Andy Trotter said: "Our concern would be if the demonstration turns into riots or damage.
"We are anxious and we have concerns that a number of organisations may well come along."
The loose network of protests is expected to begin with a march in London on Saturday followed by a faith-led vigil on the eve of the event.
Caat says it will launch "non violent direct action" as delegates turn up during the week.
However, some fringe groups say they are planning to set up roadblocks to prevent people reaching the event.
Some 1,600 officers are expected to be policing the event at a cost of £1m.
Mr Trotter justified the expense saying the experience of two years ago, when the event took place at the same time as the terrorist attacks of 11 September, meant the force had to be prepared for clashes.
Countries on the invitation list include Syria, Algeria, Pakistan, India, China and Saudi Arabia. Neither the government nor the organisers extended invitations to Iran, North Korea, Somalia, Yemen and Indonesia.
The MoD said: "Defence brings £5 billion a year to the UK and benefits between 70,000 and 100,000 jobs."
And Spearhead added: "Every exhibitor signs a compliance form saying they are not going to bring illegal weapons systems.
"That is cleared by the MoD so we know what is within UK and international law."