German police have arrested six alleged ISIS plotters amid fears they were planning an attack on a Christmas market on the anniversary of last year's atrocity in Berlin.

The Syrian men were arrested during raids involving 500 officers in the towns of Kassel, Hannover, Essen and Leipzig.

Prosecutors say they were plotting a co-ordinated attack using 'weapons and explosives'.

There are reports the target may have been a Christmas market in the north western town of Essen on December 19, but this has not been confirmed by authorities.

On the same date last year, Tunisian ISIS fanatic Anis Amri hijacked a truck to murder 12 revellers at the packed Breitscheidplatz Christmas market.

German police have arrested six alleged ISIS plotters amid fears they were planning to launch a 'coordinated attack' on a Christmas market (file picture)

In December last year, 12 were murdered when an ISIS fanatic ploughed a lorry through Berlin's Breitscheidplatz Christmas market

The six suspects were identified as Syrian citizens aged between 20 and 28 who arrived in Germany as asylum-seekers between December 2014 and September 2015. Their names were withheld under strict German privacy laws.

The men, who are allegedly members of ISIS, are accused of 'having prepared an attack with weapons and explosives on a public target in Germany.'

Police are understood to have acted on a tip off from refugees in the country after they heard of the plot.

Prosecutors refused to give further details, citing the ongoing investigation, but said the men had not yet finished preparing their attack.

Germany remains on high alert after a year of terror-related bloodshed in 2016.

Large metal gates with concrete bases have been put up near the entrance to London's Winter Wonderland in Hyde Park

A large concrete block sits on the pavement outside an entrance to Edinburgh's Christmas market to stop vehicles smashing their way in and harming visitors

Security gates are erected at the Christmas market in Manchester as the authorities increase security to keep visitors safe

Guests flock into Winter Wonderland in Hyde Park under the watchful eye of security guards as safety precautions are beefed up around the popular Christmas festival

In December last year, 12 were murdered when an ISIS fanatic ploughed a lorry through Berlin's Breitscheidplatz Christmas market.

Concrete blocks have been installed around Christmas markets at several central squares in the capital this year before the festive season opens in a couple of weeks.

In July 2016, a 27-year-old Syrian detonated an explosive device outside a music festival in the southern town of Ansbach, killing himself and wounding 15.

Last month, police arrested a 19-year-old Syrian suspected of preparing an Islamist terror attack in the country using 'powerful explosives'.

Domestic security officials estimate there are some 10,000 radical Islamists in Germany, with roughly 1,600 among them suspected of being capable of violence.