Members of the anti-immigration party Alternative for Germany (AfD) on Sunday backed an election manifesto that says Islam is not compatible with the constitution and calls for a ban on minarets and the burqa.

Set up three years ago, the AfD has been buoyed by Europe's migrant crisis, which saw the arrival of more than one million, mostly Muslim migrants, in Germany last year. The party has no lawmakers in the federal parliament in Berlin but has members in half of Germany's 16 regional state assemblies.

Opinion polls give AfD support of up to 14 percent, presenting a serious challenge to Chancellor Angela Merkel's conservatives and other established parties ahead of the 2017 federal election. They rule out any coalition with the AfD.

In a raucous debate on the second day of a party congress, many of the 2,000 members cheered calls from the podium for measures against "Islamic symbols of power" and jeered a plea for dialogue with Germany's Muslims.

"Islam is foreign to us and for that reason it cannot invoke the principle of religious freedom to the same degree as Christianity," said Hans-Thomas Tillschneider, an AfD lawmaker from the state of Saxony-Anhalt, to loud applause.

Merkel has said freedom of religion for all is guaranteed by Germany's constitution and has said on many occasions that Islam belongs to Germany.

"Islam is not part of Germany"

Up to 2,000 left-wing demonstrators clashed with police on Saturday as they tried to break up the first full AfD conference. About 500 people were briefly detained and 10 police officers were lightly injured, a police spokesman said.

The chapter of the AfD manifesto concerning Muslims is entitled "Islam is not a part of Germany". The manifesto demands a ban to minarets - the towers of a mosque from where the call to Muslim prayer is made - and the burqa, the all-encompassing body garment worn by some conservative Muslim women.

Germany is home to nearly four million Muslims, about five percent of the total population. Many of the longer established Muslim community in Germany came from Turkey to find work, but those who have arrived over the past year have mostly been fleeing conflicts in Syria, Iraq and Afghanistan.

Last month the head of Germany's Central Council of Muslims likened the AfD's attitude towards his community to that of Adolf Hitler's Nazis towards the Jews.

Although the AfD aimed to broaden its political agenda during the congress, members hardly debated on domestic issues, such as taxation and social welfare.

The party's leadership has proposed the introduction of an income tax bracket system and the abolition of inheritance taxes, which experts say would benefit high earners.

The head of Germany's DGB confederation of trade unions, Reiner Hoffmann, sharply criticised the AfD's programme.

"Their alternatives are nothing but simple, dull and inconsistent," Hoffmann said in a speech at an DGB event in Stuttgart to mark Labour Day.

He said the AfD was not only conducting a hate campaign against refugees, but also aiming for a tax policy that was against the interests of workers.