The former Ukip leader Nigel Farage has endorsed the far-right Alternative for Germany (AfD) in the runup to the German federal elections, saying it would be a “historic achievement” if the party entered the Bundestag.

“For the first time in modern history, there will be a voice of opposition in German parliament,” the South East England MEP told an audience of AfD supporters at a closed event at the Spandau Citadel, a renaissance fortress in west Berlin.

The AfD, which was founded as an anti-euro party but has since adopted a more strongly nationalist agenda, is currently polling between 8% and 11%, with just over two weeks until Germany goes to the polls. If those figures hold, it will become the first overtly nationalist party to enter the Bundestag since the Deutsche Partei, which dissolved in 1960.

The event was hosted by AfD MEP Beatrix von Storch, who claimed it had to be held inside the historic fortress for security reasons. Farage told his audience: “I am not here to tell you what to do,” but went on to dismiss Angela Merkel’s decision to keep open Germany’s borders at the height of the 2015 refugee crisis as the “worst decision by any leader in modern political history” and called the Social Democrat candidate, Martin Schulz, “a fanatic”.

Farage complained that Brexit was not playing a large enough role in the German election campaign, noting that neither Merkel nor Schulz had brought up the issue in last Sunday’s TV debate.

Farage, who resigned from Ukip in the wake of Britain’s 2016 vote to leave the European Union, called on AfD supporters to lobby Merkel to start talking about a free trade deal with Great Britain over the heads of Brussels. He said: “Merkel needs to know that unless she tells Brussels to come to a common-sense accommodation, then she will be putting the interests of Brussels above the interests of common people.”

Surveys among business associations and polls of the German public have consistently shown little appetite for opening free trade talks directly with the UK. One recent poll, carried out on behalf of public broadcaster ZDF, showed 84% of the German public opposed the EU making concessions to Great Britain during the current negotiations.

Asked in a press conference before his speech if he therefore advocated that Merkel should ignore the views of the German people, Farage responded: “Of course they don’t want to make concessions, because they have not had the debate.” Business associations, he said, “did not represent the businesses themselves”.

The AfD was founded by a group of German economists in early 2013, largely in protest against bailout payments to Greece during the eurozone crisis. Founding leader Bernd Lucke and other members of the party’s liberal wing split from the party in July 2015.

The party took a further lurch to the right last year when its leader in the state of Thuringia, Björn Höcke, called in a speech in a Dresden beer hall for a “180-degree turn” in Germany’s culture of commemorating and atoning for its crimes in the second world war.

When asked whether he condoned Höcke’s comments, Farage refused to comment, insisting that he was attending the rally on a “personal level” as a friend of Von Storch and had “no formal ties with the AfD”, even though the podium he was speaking on was adorned with the party’s logo.

Farage, who has a German wife, denied rumours that he had applied for German citizenship in the wake of last year’s referendum. “I can put it beyond doubt that I have not applied for German citizenship,” he said.

A crowd of around 30 American, British and German protesters demonstrated outside the venue, chanting “Nigel Farage and AfD, we’ll throw you all into the Spree”, referencing the river that flows through Berlin.