Germany's right-wing populist Alternative for Germany (AfD) party ramped up attacks against immigration and Islam as its poll ratings jumped in the final stretch of election campaigning.

The Muslim religion "does not belong in Germany", said a leading AfD candidate Alexander Gauland, who argued its "political doctrine is not compatible with a free country".

"Islamist rhetoric and violence and terror have roots in the Quran and in the teachings of Islam," he told reporters on Monday.

Among other proposals, AfD wants a ban on minarets and public calls to prayer from mosques, a ban on headscarves for teachers and students, and for imams to lead prayers only in German.

The latest polls show the AfD securing 10-12 percent of the vote, up from 8-10 percent.

The first far-right party set to enter Germany's parliament for more than a half a century has been saying it will press for Chancellor Angela Merkel to be "severely punished" for opening the door to refugees and migrants.

It has won support with calls for Germany to shut its borders immediately, introduce a minimum quota for deportations, and stop refugees bringing their families here.

Merkel's Christian Democrat alliance CDU/CSU slipped two points to 36 percent, close to the all-time low of 35 percent when the Social Democratic Party of Germany (SPD) led by Gerhard Schroeder defeated it in 1998.

Merkel's conservative alliance still commands a huge lead over the SPD of her top rival Martin Schulz, which slipped to 23 percent.

Third-largest party?

AfD, which has also called for Germany's immigration minister to be "disposed of" in Turkey where her parents come from, could become the third-largest party in the September 24 election, polls show.

The prospect of a party that has been compared with the Nazis entering the heart of German democracy is unnerving other parties. They all refuse to work with AfD and no one wants to sit next to them in parliament.

Gauland denies they are Nazis, saying others only use the term because of the party's popularity.

The other top AfD candidate Alice Weidel along with Gauland have stirred controversy while campaigning.

Gauland has argued Germany should be proud of its veterans of two world wars. And Weidel reportedly employed an asylum-seeker without paying tax, a claim she has denied.

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Founded as an anti-euro party, the AfD recorded a surge in support after it began capitalising on unease in Germany over the arrival of more than a million asylum-seekers since 2015.

Its members now sit in 13 of 16 state assemblies and, eyeing the national parliament, have plastered towns and cities with posters carrying the slogans "Burqas? We prefer bikinis" or "New Germans? We make them ourselves!"

Its supporters have loudly disrupted Merkel's rallies, where they loudly jeer, boo and whistle in a bid to drown her out.