Comedian Jeremy Hardy, himself banned, suggested Labour was "rigging the election" to stop Jeremy Corbyn.
Andy Burnham's campaign claimed there could be "several thousand Tory infiltrators" planning to vote.
Mr Corbyn, Mr Burnham, Yvette Cooper and Liz Kendall are all standing.
Polls suggest Mr Corbyn is the frontrunner.
New regulations allow members of the public to sign up to vote as a "registered supporter" for Â£3.
But some have been told their vote will be discounted amid concerns non-Labour supports had registered to take part.
"The Labour Party might be trying to invite a legal challenge so that they can say 'let's just scrap the election'," Mr Hardy said. "They are so desperate that I wouldn't be surprised."
Former Labour MP Andrew MacKinlay also accused the party of misusing canvass returns - collected at election time to gauge the level of support for candidates - to find people who have previously voted for other parties.
He said the exclusion process had been "arbitrary" and "unfair".
"The Labour party has got to stop today doing this," he added.
But a Labour spokesman said the party was confident "the processes of verification and handling applications are compliant with the Data Protection Act."
He added: "All applications to join the Labour Party as a member, affiliate or supporter are verified and those who are identified by our verification team as being candidates, members or supporters of another political party will be denied a vote."
Mr Burnham's campaign, meanwhile, said they had concerns about "potential Tory infiltration on a large scale."
Michael Dugher, who is chairing his campaign, wrote to Labour's general secretary calling for a "urgent meeting" of candidate teams to discuss the issue.
He claimed "a Conservative MP, Conservative media commentators, and Conservative councillors all having been rejected".
He added: "This suggests the 121,000 registered supporters could include several thousand Tory infiltrators, as well as supporters of other parties seeking to have a vote in the election."
Also on Friday, Mr Corbyn has said he will formally apologise on behalf of the Labour Party for taking the country to war with Iraq if elected leader.
Mr Corbyn told the Guardian the party would "never again flout the United Nations and international law".
Labour says it has "a robust system" to stop "malicious applications" after claims individuals who have registered to vote in the leadership race are being unfairly banned from taking part.