Sarah Waldron of the Campaign Against the Arms Trade claimed that the event would welcome "some of the most authoritarian regimes in the world".
"The deals done here fuel death, injury, fear and repression - yet instead of banning it, the Government helps make it happen.
"We are here to resist it, to stand in solidarity with the people facing the deadly consequences of its weapons and do what we can to stop this obscene event continuing."
More top news
Rowan Atkinson slipped back into character for the 25th anniversary of Mr Bean and took his mini for a spin outside Buckingham Palace.
A woman believed to be in her 60s has bee killed after being hit by a lorry in Peckham High Street, police said.
Researchers from Imperial College London believe they have found a cure for motion sickness.