German police sealed off a major shopping centre in the central city of Essen on Saturday, citing the threat of a terror attack, with media reports suggesting a link to the Islamic State group.
The country is on high alert following scenes of carnage at a Christmas market in Berlin in December, when an IS jihadist rammed a truck into a crowd of pedestrians, killing 12 people.
The German domestic security agency BfV believes the IS group was "almost definitely" behind the threat, local media reported.
According to the Bild daily, IS called for an attack and got a message to Syrian supporters in the Essen region to attack a shopping centre on Saturday.
Security services quoted by Bild described the threat as a potential multiple suicide bombing at the mall, one of the biggest in the country.
"The shopping centre will be closed all Saturday due to security concerns. The police have concrete information regarding a possible attack," local police said in a statement published on social media.
Local car parks and the underground train station were also closed.
Though there was no announcement of arms or explosives being found, police said two men had been picked up for questioning.
Both men were arrested in the town of Oberhausen near Essen but later police said in a statement that the pair "are not suspects" in the case.
- 'Major operation' -
"Many agents are deployed onsite. This is a major operation," a local police spokesman told AFP, indicating the lockdown included the 200-store Limbecker Platz in downtown Essen, nearby parking garages and an underground rail station.
Sniffer dogs were also been deployed at the site.
Essen, which is in the industrial Ruhr region, has a population of approximately 500,000.
The police said they had been alerted to the threat by "another department" but no German agency has confirmed if it was involved.
Interior ministry spokesman Tobias Plate told AFP that the operation was being handled by the local police force but added that his ministry was in "constant touch" with the GTAZ, a joint counter-terrorism centre used by 40 internal security agencies.
German authorities have been on alert since the deadly Christmas market attack in Berlin.
A Tunisian failed asylum seeker, Anis Amri, rammed a hijacked truck into the crowded market on December 19, before being shot dead four days later by police in Italy.
Last July, a German-Iranian teenager who police say was obsessed with mass murderers, shot dead nine people at a Munich shopping mall before turning the gun on himself.
Fears of another attack rose on Thursday when a 36-year-old paranoid-schizophrenic man from Kosovo rampaged through Duesseldorf railway station with an axe, wounding nine people. Police have ruled out a terrorist motive for that attack.
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.
IS has claimed responsibility for attacks in Germany in the past year, including the murder of a teen in Hamburg, a suicide bombing in Ansbach and an axe rampage on a train in Wuerzberg that injured five.