Scottish UKIP candidate Mark Meechan was a prominent user of an online forum that contained racist language and threats against ethnic minorities.
The forum was closed down by its host, US-based gaming community site Discord, following inquiries by the BBC.
The chat group, which was littered with racist and Islamophobic terms as well as support for neo-Nazi groups, was promoted from Mr Meechan's Twitter.
Mr Meechan said the forum "operates on the principle of free speech".
The 31-year-old YouTube blogger, from Coatbridge, was convicted last year of posting a video of his girlfriend's pug lifting a paw when he said "gas the Jews" or "Sieg Heil".
It was described by a sheriff as anti-Semitic and racist but Mr Meechan denied he was a racist and said his conviction set "a very dangerous precedent" for free speech.
Mr Meechan is on the list of UKIP candidates for the European Parliament elections on 23 May.
His Twitter profile, on which he promotes his newly-launched political career, pointed his 200,000 followers towards the Discord forum, which required users to register.
It warned potential members that material in it was offensive and asked them to consent to viewing the different discussion threads.
Discord said that, after being approached for comment by the BBC, it had taken down the forum for violating its terms of service.
These prohibit "hatred, harassment, threatening messages, calls to violence or any illegal activity".
Mr Meechan often referred to the forum as "my Discord".
He told the BBC he did not own it and was not the chief moderator.
On the forum, Mr Meechan, who posts under his online persona Count Dankula, used racist language, including the N-word.
He also posted memes about black people, including one implying they were thieves.
It shows a picture of film character Indiana Jones about to steal a golden relic in a cave and is captioned: "Black people. Anything that doesn't belong to them."
Another meme shows Labour MP Diane Abbott and her supposed reaction when white people do not apologise for slavery "at the beginning of every sentence".
The BBC has decided not to publish the images.
In response to the BBC, Mr Meechan said that he was a comedian.
He said: "I posted an offensive meme, something I do all the time, I tell offensive jokes and do offensive comedy… do you really think anyone is going to be surprised at the fact I post offensive memes?"
He said the meme about Ms Abbott "makes fun of her for statements she has made in the past, not for the colour of her skin".
Mr Meechan and the other moderators in the forum's hierarchy set the rules of the forum and decided what posts from its almost-9,000 members were acceptable.
But the BBC has seen various discussion threads where users frequently used threatening and racist terms about ethnic minorities, including one which posted offensive terms to suggest they should kill all Jews, enslave all black people and torture all gay people.
In one thread, users posted about gassing black people and "nuking" Muslims.
Others use homophobic language, derogatory terms about disabled people and display apparent support for neo-Nazi groups, including the violent American neo-Nazi group Atomwaffen Division.
It has been linked to five murders in the US, and espouses a violent white supremacist ideology.
One user writes: "Atomwaffen division did nothing wrong. The degenerates got what they deserved."
Another user posts about a UK neo-Nazi organisation modelled on Atomwaffen Division, known as Sonnenkrieg Division, saying they wished the group "would actually [do] something".
Two members of Sonnenkrieg Division have been convicted of terror offences, including circulating material suggesting that Prince Harry should be shot as a "race traitor" for marrying a woman of mixed race.
Both groups have previously been revealed by the BBC to communicate via Discord.
Last month's deadly terror attacks on mosques in Christchurch, New Zealand, were also discussed on the forum.
One user posted that the killer "proved white supremacy". It says he got "a lot more done with a lot less firepower" than Islamist attackers.
Mr Meechan can be seen on the forum warning users that sharing the video of the Christchurch shootings will result in the forum being taken down by Discord.
Before the forum was closed down on Monday night, Mr Meechan said: "We let people say what they like, if there is anything that directly breaches [Discord's terms of service] or breaks the law, it is removed by our moderator team [which] consist[s] of Jewish people, black people, Asian people, Muslim people, trans people etc."
Meechan said that the forum was "extremely diverse" and that "everyone" posts offensive material, including about white people.
The forum logs show that moderators have temporarily banned users for using racist and other offensive language, but large volumes of it had gone unchecked until it was shut down.
Discord is a chat platform which has been likened to Slack or Skype for gamers, but has also become popular with other groups looking for secure chat rooms.
Discord said it did not read the billions of messages posted on the platform "one-by-one" and that violations made up a "tiny percentage" of the material shared via the site.
UKIP is the only party standing up for issues that affect young people. Such as freedom of speech, democracy and a non-partisan education system, where all views should be encouraged. pic.twitter.com/ysdWNMgoWg— UKIP (@UKIP) April 30, 2019
UKIP has not yet responded to the BBC's requests for comment on the forum.
On Tuesday afternoon the party posted a tweet with a picture of young activists and candidates, featuring Mr Meechan at the front of the group.
The details of Mr Meechan's forum come after Buzzfeed news reported fellow YouTuber and UKIP European election candidate Carl Benjamin had directed supporters to a Discord forum containing similarly racist material, and that Mr Benjamin had used racial slurs in YouTube videos.
Last year, Mr Meechan, who has 500,000 YouTube followers, was fined £800 at Airdrie Sheriff Court for uploading footage of him prompting his girlfriend's dog to raise its arm in the style of a Nazi salute by repeatedly saying "Seig Heil" and "Gas the Jews".
Mr Meechan failed in his attempt to appeal against the conviction.
The newly-launched BBC Scotland channel had recorded episodes of a discussion programme featuring Mr Meechan and others with controversial views, but later pulled the two episodes in which he featured.
Mr Meechan said he was "entering politics as a staunch advocate for freedom of speech and freedom of expression", but UKIP did not know about the Discord forum "because it wasn't related to the party".
He added: "It's a large closed group where people come to hang out and chat.
"The server acts as a community where people from all walks of life can come and speak freely..."