BERLIN - German authorities are on guard against threats of armed attack on civilians of the kind that killed 166 in the Indian city of Mumbai in 2008, Interior Minister Thomas de Maiziere said on Thursday.

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Police are taking added precautions, de Maiziere said, a day after Germany tightened security measures citing threats from Islamist militants.

"What we are basically preparing ourselves for, is that terrorists, coming from abroad, commit an attack soon after arrival, without warning, in a building or public place, knowing that they may not survive," de Maiziere told broadcaster ZDF.

"In shop talk, we'd call that something similar to what happened in Mumbai," he added.

On Wednesday, Germany said it had strong evidence Islamist militants were planning attacks in the next two weeks, and it ordered security at potential targets such as train stations and airports to be tightened.

Security officials in Berlin said on condition of anonymity then that three scenarios existed -- one involving senior al Qaeda leader Younis al Mauretani, another involving the Mumbai-style attack, and a third pointing to sleeper cells already in Germany.

As de Maiziere spoke, German police were on route to to Namibia to aid a separate investigation into a suspect package found at Windhoek airport during security checks for a German tourist flight to Munich.

De Maiziere said he was waiting for results of the investigation before he could say what had happened or if there had been any connection to other threats.

"We don't know yet what happened," he said. "By all means there is evidence it could have been a test, but no confirmation," he said.

In Germany, threats also existed from lone "fanatic operators" and not only a Mumbai-style group attack, de Maiziere said, adding that such large-scale killing would be prevented.

"God willing, as long as we work well, this attack scenario will not happen in Germany, and the measures we are taking -- visibly and behind the scenes -- are helping," he said.