Ciaron O'Reilly, 52, attended the event with the London Catholic Worker group, one of a number of organisations to participate.
He said: "I think the persecution of Bradley Manning and Julian Assange are the anti-war show trials of our generation. These people risked their lives and liberty to expose the truth about the wars (in Afghanistan and Iraq)"
Paul Davis, 25, a stuntman from Surrey, also attended with his friends.
He said: "Whistleblowers should not be persecuted for exposing the crimes of their governments and armies. Whistleblowers has become a by-word for victims and martyrs."
About 30 people were outside the embassy at around 4pm when the protest ended. After repeated shouts of "Free Bradley Manning", some campaigners moved on to the Ecuadorian Embassy to support Mr Assange.