Police are investigating an assault complaint relating to Labour member Walter Wolfgang's ejection from the party conference last week.
Mr Wolfgang was removed for heckling
Mr Wolfgang, 82, from London, was physically removed from the Brighton Centre for shouting "nonsense" during a speech by Foreign Secretary Jack Straw.
A Sussex Police spokesman said a "complaint of assault in relation to the incident" was being looked at.
But it was not revealed who had made the allegation.
In a separate development, Sussex Police has apologised to Mr Wolfgang, a refugee from Hitler's Germany, for quoting anti-terrorism powers when officers questioned him outside the Labour conference.
Mr Wolfgang, a Labour party member for 57 years, was stopped by officers when he tried to re-enter the building.
They wrongly said they were using powers under Section 44 of the Terrorism Act to "detain" him.
In a letter to Mr Wolfgang, Sussex assistant chief constable Geoff Williams said: "This was a genuine mistake and I am writing to you to apologise for the distress this may have caused you."