Police ordered a shopping centre in the German city of Essen not to open on Saturday after receiving credible tips of an imminent attack.
The shopping centre and adjacent car park stayed closed as about 100 police officers, many armed with machine pistols and bullet-proof vests, positioned themselves around the compound to prevent anyone from entering. Several officers scoured the interior to bring out early morning cleaning staff.
Police spokesman Christoph Wickhorst said: "As police, we are the security authority here and have decided to close the mall."
He said police had been tipped off by other security agencies late on Friday. He declined to provide further details, but police later said there had been concrete indications for an attack on Saturday.
Essen police said they were questioning a man and had searched his apartment in nearby Oberhausen.
A second man was detained at an internet cafe in Oberhausen in the afternoon and police also increased their presence at a shopping centre, the Centro, in that city, the German news agency dpa reported.
In Essen, the shopping centre at Limbecker Platz square was closed for the entire day. It is one of the biggest shopping centres in Germany with more than 200 stores and attracts up to 60,000 people on a Saturday, according to its website.
Last year, three people were injured in an attack on a Sikh temple in Essen by radicalised German-born Muslim teenagers.
Germany has been on the edge following a series of attacks in public places over the past year.
When a man went on a rampage with an axe on Friday night at Duesseldorf's main station and injured 10 people, about 600 police officers were deployed to the scene, even though it later turned out that he had no links to extremists, but was suffering from psychological problems.