It was a warm spring day in Duisburg, a rusty industrial hub in Western Germany, and Alan Imamura, a member of the City Council, was chatting with constituents in a shop-lined pedestrian mall on the city’s impoverished north side.

Until recently, Mr. Imamura said, he was not welcome in places like this. That is because he is a leading local figure in the Alternative for Germany, known as the AfD, a far-right party whose national organization was recently declared an extremist group by the country’s domestic intelligence service.

Much of the AfD’s support comes from the former East Germany. But in recent years, it has developed a beachhead in parts of Western Germany. During February’s federal elections, several neighborhoods in Mr. Imamura’s district gave the AfD some of its best results in the country, coming close to 40 percent of the vote.

“It’s so different,” he said. “You would not imagine, five years ago — when I put up some posters, people spat on me. And today the people, they say, ‘Finally.’”

The AfD emerged over a decade ago around skepticism against the euro, but it soon morphed into a party built on the denigration of immigrants and refugees, one of the reasons it was designated as extremist.

A confidential, 1,018-page report by the domestic intelligence service, which was not released but was reviewed by Der Spiegel magazine, documents what it called “an entrenched xenophobic mind-set” within the “top leadership structures of the AfD.” For example, Bjorn Höcke, who leads the AfD in the Eastern state of Thuringia, has repeated Nazi-era slogans and called for “large-scale” deportations with “well-tempered cruelties.”