At an event called "Why socialists can't leave Labour to the centrists", members were told to prepare to take action against Sir Keir.
Steve Howell, who advised Mr Corbyn when he was in charge, described the treatment of his former boss by the current leadership as "very shabby".
"Like tens of thousands of members, I'm fuming at the treatment of Jeremy Corbyn by Keir Starmer," he told the meeting.
Mr Corbyn had the whip removed from him after comments he made in response to the publication of the Equality and Human Rights Commission report into anti-Semitism in Labour last autumn.