The posters show Juncker and Soros laughing above a slogan: "You too have a right to know what Brussels is preparing".

Hungary alleges that the EU and Soros are actively supporting illegal immigration, a charge branded "a ludicrous conspiracy theory" by the Commission.

"We have started the first stage of our 'reality campaign', our activists have already covered up the lies on the posters with stickers," the liberal opposition Momentum party said on its Facebook page, along with photos of the action.

Momentum spokeswoman Maria Gerencser told AFP that the aim of the campaign was to invert the slogan, "drawing people's attention to the real facts that Hungarians have a right to know".

Instead of the government's message about the threat from immigration, the stickers added by Momentum say: "Hungarians have to the right to know there is a shortage of 4,000 schoolteachers and 400 doctors", as well as the claim that "Viktor Orban has become Hungary's richest man".

"We think that the biggest danger for Hungarians isn't the EU or Soros but corruption, low salaries, the labour shortage and emigration," Momentum said.

Critics allege that Orban's family and friends have benefited from corruption under his administration, a charge denied by the government.

As well as the posters, the campaign includes newspaper and TV adverts and has drawn the ire of several prominent EU politicians, including those belonging to the centre-right European People's Party grouping -- to which Orban's Fidesz party also belongs.

Juncker too hails from the EPP camp and said last week when the campaign was launched that Orban's "place is not in the EPP", echoing other calls in recent months to expel Fidesz from the grouping.

Momentum, the small centrist party that announced the counter-campaign, was founded relatively recently and has no MPs.

It launched a similar action against a previous anti-EU campaign by the government.