Protesters stormed an armed forces recruiting centre to stage an anti-war demonstration.
Police sealed off Queen's Road, Brighton, as more than 20 supporters of a group called No War But The Class War occupied the building to cause disruption.
Most of them left after a few minutes to continue protesting outside but three remained longer.
The three also climbed scaffolding outside the building and displayed a banner saying Sabotage The War Effort.
Acting Chief Inspector Helen West said the protest was peaceful but the main road had been closed for health and safety reasons.
A protester who gave his name as Jamie said: "In declaring a war against terrorism, the ruling class has really declared a war on workers across the world."
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