A German politician vying to become the first state premier for the rightwing populist Alternative für Deutschland in elections this Sunday took part in a neo-Nazi rally in Athens in 2007, documents leaked to the media show.

Andreas Kalbitz, 46, is the AfD’s lead candidate for Brandenburg, where polls suggest the party is competing with the centre-left Social Democratic party (SPD) in the race to become the state’s strongest political force.

A highly influential figure on the AfD’s overtly nationalist wing, Kalbitz’s influence in the party is likely to increase following state polls in Brandenburg, Saxony and Thuringia this weekend, and he has emphasised his strict rejection of anti-democratic ideas and organisations.

However, a report by the German embassy in Athens, published on Friday by Der Spiegel, named Kalbitz as one of 14 German neo-Nazis who travelled to the Greek capital in January 2007 for a rally organised by the Patriotic Alliance, a short-lived ultranationalist party formed by members of Golden Dawn.

Among the other members of the German delegation were leading figures in the NPD, founded as a successor to the German Reich party, including its leader, Udo Voigt, Der Spiegel reported.

The group drew the attention of the local police and the German embassy because they had hung a flag bearing the swastika from the balcony of Hotel Solomou in central Athens. The hotel’s entrance and balcony were firebombed later that evening. The embassy believed the attack had been carried out by members of the anarchist scene.

When approached with the information by Der Spiegel, Kalbitz confirmed he had been in Athens in 2007 but said he had not been present at the hotel during the firebombing “and the sequence of events connected to it”.

The Bavarian-born AfD politician claimed to have been an unenthusiastic participant in the rally. “Assessing this event retrospectively, it did not wake further interest or approval from my behalf, neither in its political goals nor the selection of those participating.”

Polls this month showed the AfD as the strongest party in Brandenburg, the state that surrounds the capital, Berlin, but more recent surveys give a slight advantage to the SPD, which has governed there since German reunification.

In neighbouring Saxony, polls show the AfD to be floundering in the final stretch of the campaign, on track for second place on 24.5% behind a resurgent Christian Democratic Union (CDU) on 32%.

The revelation of Kalbitz’s neo-Nazi links is the latest detail to undermine efforts to lend the rightwing populist party a respectable face. This month the newspaper Die Welt unearthed two documentaries about the second world war listing Kalbitz as a scriptwriter alongside his late father-in-law Stuart Russell, a UK citizen who served in the British army in Germany and later settled near Paderborn.

Thomas Weber, a history professor at the University of Aberdeen, described one of the films, Hitler: The Unknown Soldier, 1914-1918, as a “skilful glorification of Hitler”, which uncritically digested National Socialist propaganda and portrayed the first world war as the result of an “evil alliance” of Jews and Marxists.

Kalbitz told Die Welt he had helped with the script for the film because the father of his wife was not a native speaker, and he insisted Russell “was not a rightwing radical or extremist”.