Members of a far-right German group arrested last week were plotting "shocking" large-scale attacks on mosques similar to the ones carried out in New Zealand last year, a government spokesman said on Monday.

Officials said investigations into 12 men arrested in police raids across Germany on Friday had indicated they planned big attacks, following media reports over the weekend the group aimed to launch several simultaneous mass-casualty assaults on Muslims during prayers.

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"It's shocking what has been revealed here, that there are cells here that appear to have become radicalised in such a short space of time," interior ministry spokesman Bjoern Gruenewaelder told reporters at a Berlin news conference.

According to media reports, the group planned to use semi-automatic weapons to mirror attacks in Christchurch last March in New Zealand in which 51 people were killed at two mosques.

"It is the task of the state, and of course of this government, to protect free practice of religion in this country, with no reference to what religion it might be," Chancellor Angela Merkel's spokesman Steffen Seibert said.

"Anyone practising their religion in Germany within our legal order should be able to do so without being endangered or threatened".

The alleged leader of the far-right group, which was known to the authorities and whose meetings and chat activity had been under observation, detailed his plans at a meeting organised with his accomplices last week.

Investigators learned about that meeting from someone who had infiltrated the group, reports said.

Prosecutors said they had launched early morning raids to determine whether the suspects already had weapons or other supplies that could be used in an attack.

The country's underground far-right scene is under increased scrutiny since the murder of conservative local politician Walter Luebcke last June and an October attack on a synagogue in eastern city Halle.

Der Spiegel reported that police currently list 53 people belonging to the extreme right as "dangerous" individuals who could carry out a violent attack.

Germany's domestic intelligence agency estimates there are about 24,100 "right-wing extremists" in the country, about half of whom are potentially violent.

According to the government, there were nearly 9,000 attacks by far-right groups and individuals in the first half of 2019 - an increase of nearly 1,000 compared with the same period the year before.

Meanwhile, there are growing concerns about the far right's political influence in Germany, following a recent scandal that saw mainstream parties collaborate with the nationalist Alternative for Deutschland (AfD) in a local election - a move Chancellor Merkel called "unforgivable".