Anti-Islam campaigner Anne Marie Waters, the founder of the Sharia Watch group, claimed "tactics and manipulation" have been used to try and stop her winning the party's contest.
A number of UKIP's MEPs are expected to quit if Ms Waters is elected their new leader, amid concerns she and her supporters are staging an entryist bid to take over the party.
Ms Waters, a former Labour activist, was barred from standing for UKIP in June's snap General Election, despite being selected by local party members.
But party bosses allowed her to stand in this summer's leadership contest, sparking a challenge against the decision from fellow candidate Henry Bolton.
At the party's London hustings event on Monday night, Ms Waters launched a furious attack on her rivals.
She said: "An attempt was made to remove me from the ballot paper and I've got no doubt whatsoever in my mind that there are other candidates, whether here tonight or not, who would have been more than pleased to see that succeed.
"Now remember that trying to silence me is also trying to silence every supporter I have in this party.
"And that's quite a lot, which is why the attempts at silencing has taken place."
Ms Waters, who has called Islam "evil", previously launched a UK branch of German anti-Islam movement PEGIDA together with former English Defence League leader Tommy Robinson.
She accused her UKIP rivals of "frightening people away from listening" to her.
"This has been going on far too long, this is EU-type behaviour," she added.
"This is using tactics and manipulation to attempt to get people to vote the way you want them to vote.
Eleven candidates originally stood in the race to replace former party leader Paul Nuttall, who quit after the General Election.
The number thinned on Monday when three dropped out - including UKIP's Scottish MEP David Coburn - to back Yorkshire and North Lincolnshire MEP Jane Collins.
Under the banner "UKIP United", the four will hope to stop the large field allowing an "authoritarian" leader such as Ms Waters from winning the contest.
A spokeswoman for Ms Collins said: "These people are not what the party should be about."
Monday's hustings featured questions on each candidate's views on female genital mutilation (FGM), the burka, Islam, border controls and UKIP's future funding.
Former leader Nigel Farage attended the debate halfway through.
Other leadership candidates include deputy leader Peter Whittle and London Assembly member David Kurten.
The new leader will be announced at UKIP's annual conference in Torquay on 29 September.
