Peace campaigners who have appeared in court charged with obstructing access to a London arms fair will argue that their actions were necessary to prevent crimes being committed using the weapons on sale.

Five men and three women appeared before Stratford magistrates court accused of obstructing the highway outside the Defence and Security Equipment International (DSEI) exhibition in east London last September.

Their protests stalled lorries and military vehicles attempting to gain access to the biennial event, one of the world’s biggest arms fairs, in the ExCeL centre in London’s Docklands.

None of the defendants contest the prosecution claim that their protests blocked the highway outside the venue. However, they claim their actions were an attempt to halt greater crimes being committed by buyers attending the arms fair, including delegations from Saudi Arabia, Israel, Bahrain, Pakistan and Azerbaijan.

Ian Brownhill, one of the protesters’ barristers, said of their defence: “It is relatively novel and the higher courts have yet to decide whether such a defence is sustainable.”

Isa Al-Aali, 21, a Bahraini human rights activist who was granted asylum in the UK after the crushing of Arab spring protests in his home country, was among the defendants.

Police remove protesters and journalists from the entrace of the trade fair. Photograph: Mark Kerrison/Demotix/Corbis

He said outside court: “I’ve been arrested many times and tortured, and so have other Bahrainis. The dictatorship uses arms to kill the people and because of that I was against the arms sales. My family are still in Bahrain and their lives are in danger.”

A second defendant, Angela Ditchfield, said before the trial began: “The state is acting to stop protest against genocide and mass displacement of people rather than acting to stop the criminals from doing it and that’s really upsetting.

“It’s not necessarily shocking any more because they have done so much of it, but there is this huge weight of state funding for supporting the arms trade and getting rid of anyone who gets in the way.”

She said police at the demonstration had asked if there was anything they could do to persuade her to move without arresting her. “I said if you will check that there are no illegal weapons in this vehicle or any of the vehicles going past I will move,” Ditchfield added.

“They just laughed and said we can’t do that, we are not that type of police, that’s not our remit. But there is no one there whose remit it is to stop that.”

Organised every two years by Clarion events, DSEI brings governments together with many of the globe’s biggest arms companies. Protesters say some of the world’s most oppressive regimes are represented.

Chris Roper, an activist with Veterans for Peace who had previously worked in the arms trade, was among about a dozen protesters outside court supporting the defendants. He said: “When you are actually working for a firm that is producing weapons you are told it is for defence and making the world a safer place. But when you see what the weapons do, you realise that the truth is very different.

“The workers don’t realise. You don’t know what those weapons are going to be used for and then they are sold and the people who sell them don’t know what they are going to be used for. And then they are sold on to another country and used to repress the people of that country.”

A protester makes preparations for the day of action behind a row of mock CS gas canisters. Photograph: Mark Kerrison/Demotix/Corbis

The defence intends to call expert witnesses including Andrew Feinstein, the director of Corruption Watch UK; Oliver Sprague, the programme director of arms control and policing at Amnesty International UK; and Sayed Ahmed al-Wadaei, the director of advocacy at the Bahrain Institute for Rights and Democracy.

Defendants also intend to give personal testimony of their reasons for opposing DSEI, including a personal account by Aali of the torture he suffered during the Bahrain protests.

The prosecution indicated it would argue that the defence of trying to stop crimes being committed was invalid.

Aali, Ditchfield, Lisa Butler, Thomas Franklin, Javier Gárate Neidhardt, Susannah Mengesha, Luis Tinoco Torrejon and Bram Vranken all deny wilful obstruction of the highway. The case continues.