Eileen Means said the cause of "hundreds" of suspensions was unclear as the party had failed to notify them.
She said the process was "extremely unfair" as those affected "don't know what offence they've committed".
A Labour spokesman said those suspended will also have their votes cancelled in the party's leadership elections.
The party added it has a "robust validation process" to ensure all votes meet "Labour Party rules".
Labour's ruling body, the National Executive Committee (NEC), issued new social media guidelines which party members had to sign up to this week.
Ms Means believes most of the suspensions - from the 6,700 strong Bristol membership - were down to infringements of the new guidelines.
She described the suspensions as "too heavy" for some of the "so-called offences".
"There's a grave suspicion that it is a purge and a lot of members are using that term," she said.
"I think they are suspending anybody who does abuse anybody [else] but there's no doubt that people who support [Jeremy] Corbyn outnumber the people who support Owen [Smith] - certainly in Bristol."
During the past month, three Labour councillors - Harriet Bradley, Hibaq Jama, and Mike Langley - have been suspended, pending further investigations, costing the party its majority on the city council.
In an open letter to the NEC, a number of Bristol Labour members protested against the councillors' suspensions claiming the move had "done serious damage to the local party".
It said the councillors were given "so little information" it was "nearly impossible" to determine why they were suspended.
In a statement, Labour said it would not comment on individual cases but confirmed decisions "about individual members" was taken by the NEC.
"Members who have been contacted by the Labour Party to confirm they are not deemed eligible to vote can contact the party to obtain the evidence considered by the NEC," it said.
Two hundred Labour members in Bristol have been suspended for allegedly breaking party rules, the chair of the local party has claimed.