Six people have been arrested for aggravated trespass after they blocked the entrance to what was described as an "arms fair" in Cardiff.
Defence Procurement, Research, Technology and Exportability 2018 is taking place at the Motorpoint Arena.
The Ministry of Defence said it brings together suppliers, buyers and thought leaders in the defence industry.
However, a campaign group on Facebook said some companies in attendance supply to "brutal repressive regimes".
Stop the Cardiff Arms Fair called on people to oppose the event taking place.
In a statement, South Wales Police said it was in attendance to "facilitate peaceful protest" and "minimise disruption" in the city centre.
It added: "Six people have been arrested for aggravated trespass after an entrance to the arena was blocked preventing staff from attending their normal place of work."