A peace activist standing trial over attempts to block access to an international arms fair has told a court he acted “to uphold the rule of law” after illegal weapons were found to be on sale in previous years.

Thomas Franklin, 57, an academic from York, said he tried to block a tank from entering the Defence and Security Equipment International (DSEI) exhibition so it could not be sold to regimes that abuse human rights.

He told the district judge Angus Hamilton at Stratford magistrates court in east London that he believed the fair was also used to sell illegal torture equipment and banned weapons such as cluster munitions, and therefore it was his civic duty to try to stop it from going ahead.

“I thought I should go there to show my disapproval of what is going on,” Franklin said. “It was actually supposed to be a day of academics against arms, but I wanted to be there in body to say I object to this, to the sale of arms and especially to the sale of illegal arms.

“In every single previous arms fair, that had been found to be happening. We have evidence of that. We have parliamentary reports, we have reports from Amnesty International, we have reports from Caroline Lucas, the Green MP, listing illegal weapons being sold.”

It is believed to be the first time such a defence has been used to defend protests against an arms fair in Britain.

Franklin is one of eight defendants who deny wilful obstruction of the highway in relation to protests at the DSEI fair at the ExCeL centre in London’s Docklands last September. All have claimed they acted as they did to stop greater crimes being committed by regimes represented at the fair, including Azerbaijan, Bahrain, Israel, Pakistan and Saudi Arabia.

Franklin said he acted spontaneously after people disappeared from a seminar he had been attending as part of the demonstrations. “I went to see what they were doing. They stopped a vehicle carrying a tank. I thought I can’t just stand here, if I believe in this I have got to stop a crime taking place and I’ve got to uphold the rule of law, because the government … were clearly not doing their job. If the government is not going to uphold its laws, I felt I had to ensure that the rule of law was being upheld.”

In cross-examination, Caoimhe Daly, prosecuting, challenged Franklin to accept that the vehicle he was blocking was not committing a crime in that moment. He replied: “It was in preparation for a crime, therefore it was committing a crime because it was in preparation of a crime.

“It might have broken down and failed to kill anyone, but if I saw an armed person with a gun and stopped them and only later found out it was a toy gun I would still be right to stop them.”

Angela Ditchfield, 36, from Cambridge, said her Christian faith had compelled her to join the demonstration against DSEI. She said it was clear that arms deals were signed there which sometimes involved illegal weapons and torture implements. She had run into the road to stop a military vehicle that was being driven into the ExCeL centre.

“We thought that there was a very high probability, given the history, that there would be illegal torture weapons or advertising material for them being taken into the arms fair,” Ditchfield told the court. “I said to the police many times, if they are able to check or get a weapons inspector to check the vehicles and check that was not happening.

“I was prepared if they had actually checked for the illegal weapons, I did say that I would then move. I was concerned that they would be used to torture people in Saudi Arabia, their own citizens, Christians, political opposition. I was concerned that they were … the news was just breaking about them being used in Yemen, where they were being targeted at hospitals and schools, and they are being used to oppress, to stop protesters in Bahrain.”

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

In a joint public statement, the defendants’ campaign said: “Tools of the type promoted for sale at DSEI will be used to reinforce apartheid, to surveil and brutalise communities from Brixton to Bahrain, and to perpetuate the border regime that kills thousands every year – as European states wage a war against the refugees they helped create.

“We know that weapons of the type promoted at DSEI will be used to torture and repress people based on their political views, faith, race, gender or sexuality in places like Saudi Arabia. Sometimes the tools of oppression are literal – and they are for sale at DSEI.”

The trial began on Monday and is expected to last five days. Franklin, Ditchfield, Isa al-Aali, Lisa Butler, Javier Gárate Neidhardt, Susannah Mengesha, Luis Tinoco Torrejon and Bram Vranken all deny wilful obstruction of the highway.

The trial continues.

