The leader of an extreme Hungarian nationalist party has declared that the furore over his controversial visit to London on Sunday has made him even more determined to speak in Britain.

Gábor Vona, 35, whose party has been accused of strong antisemitic views, and of fuelling hatred against the Jewish and Roma communities, posted a diatribe on Facebook in which he said: "I wish to be there more than ever."

Vona's plans to speak at a rally at Holborn tube station, in central London, a day before Holocaust Memorial Day, have drawn condemnation and calls for him to be banned from entering the UK.

Fears are growing that his rally, allegedly organised to allow him to speak to Britain's 50,000 Hungarian expats before this year's election, could be a violent flashpoint, with members of Unite against Fascism expected to attend in protest.

Scotland Yard has made contact with Vona through his party's Facebook page to ask him about this motives and plans, according to the far-right politician, who added on the social media site: "Polish groups living in England declared that if anyone tries to attack Hungarians, they will be there and they will step up."

Vona has advocated segregation for Hungary's 800,000 Roma. His foreign policy spokesman Marton Gyongyosi has called for a register of Jewish people who allegedly pose a "national security risk". He subsequently apologised.

The party's paramilitary wing, Hungarian Guard, likened to the Nazi brownshirts for the habit of wearing traditional dress, with black boots and trousers, was banned in 2009.

Last week, a petition of 14,000 signatures was presented, at the Home Office asking Theresa May, the home secretary, to ban Vona from the UK. But sources close to the government said Vona had been careful not to make comments that could permit such a ban on the grounds of inciting racial hatred.

Last Saturday, Vona, whose party, Jobbik, is Hungary's third-largest, winning 17% of the vote and nearly 50 seats in parliament, said in a speech that his party now shunned racism.

While endorsing capital punishment and chemical castration for some criminals, he told a party meeting: "We've had enough of racism! There won't be a law in Hungary that would allow for a differentiation to be made on ethnic or religious basis between a human and another one.

"We want equality. Even the honest Gypsy will do better if Jobbik gets to govern. Not because he is Gypsy, but because he is honest."

But the London assembly member for Barnet and Camden, Andrew Dinsmore, whose constituency includes Holborn, said Vona should be banned on the basis of his party's previous rhetoric and record. "The parallels with Hitler are frightening We produced our petition in 24 hours and there has been silence from the Home Office. From what I understand, this lot are even worse than Golden Dawn in Greece."

Baroness Hussein-Ece, the Liberal Democrat spokesman for gender and equality in the Lords, also said she supported those campaigning against Vona's visit. She said: "We have a responsibility for community relations and someone like this is going to add anything to the fabric of our society. He is anti-semitic, Islamophobic and racist and there is a coaltion of views saying we do not want him in this country".

Karen Pollock, chief executive of the Holocaust Educational Trust, said: "The visit to the UK of a man who has become the face of far-right and antisemitic sentiment in Hungary on the eve of Holocaust Memorial Day is deplorable.

"Jobbik, the party which Vona leads, have expressed explicitly antisemitic views, including calling for a list of Jews in Hungary to be drawn up due to their 'national security risk'. How can such views be welcome in this country, on this day of all days?"

The Holocaust Memorial Day commemorations will be launched in the UK at King's Cross station on Monday, less than a mile from where the rally is expected to take place. On the grounds of the Auschwitz concentration camp, more than 60 members of the Israeli parliament, including the speaker in the Knesset, will gather to remember those who suffered there.