Five protesters have been arrested after a break-in at defence giant BAE System's Filton facility near Bristol.
The Al Yamamah deal involved Tornado and Hawk aircraft
Protestors describing themselves as "a group of individuals" had earlier said they planned to "occupy" the site.
They said they were protesting against the ending of a Serious Fraud Office investigation into a 1986 arms deal between BAE and Saudi Arabia.
The highly secure facility is used by scientists working on the next-generation of missile technology.
'Fight the arms trade'
Avon and Somerset Police said that four men and one woman had been arrested on suspicion of burglary, and would be questioned about the incident during the day.
A police spokesman added that the five people aged between 20 and 50 had been detained while staging a protest inside a building owned by BAE Systems.
A statement from the protesters said: "We're going to fight the arms trade until it disappears. We're going to keep on laughing at the myth that any corporation will act responsibly."
The Serious Fraud Office ended its investigation into BAE's Al Yamamah arms deal with Saudi Arabia in December 2006, on the grounds that continuing it would not have been in the wider public interest.
The Saudi government was reported to have been angered by the investigation into allegations of a slush fund for members of the country's royal family.