Bomb disposal experts were called into action twice within hours in the city centre on the eve of the Labour Party Conference.
Police cordoned off an area at Gardener Street, Brighton, at about 9am on Saturday morning when a box was discovered on the canopy of the Corn Exchange.
Bomb disposal experts were called to the scene and a controlled explosion carried out after which it was discovered the box contained polystyrene pieces.
Hours later, police cordoned off an area near the Churchill Square shopping centre and MacDonalds where a suitcase had been left.
The bomb disposal experts were called in again and carried out another controlled explosion.
A spokesman for Sussex Police said: "Because of the heightened security measures in place because of the conference we have the bomb disposal facilities more readily available to us."
The bomb disposal officers were in action in the city on Thursday night when they carried out a controlled explosion on a package discovered at the rear of the Brighton Centre.
Police later confirmed the article had been a tool box which had been left unattended.
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