Germany said on Wednesday it had strong evidence Islamist militants were planning attacks in the next two weeks and ordered security at potential targets such as train stations and airports to be tightened.
Interior Minister Thomas de Maiziere said details of the plot emerged after parcel bombs were dispatched from Yemen to US targets at the end of October, and separate postal bombs by suspected Greek militants were sent to prominent figures including Chancellor Angela Merkel.
At a news conference hastily convened at his ministry in Berlin, de Maiziere said the security threat in Germany had risen and that intelligence services had received concrete indications that attacks were planned at the end of November.
"There are grounds for concern but not for hysteria," said the minister. "We have obtained further relevant information in addition to the previous findings which ... justify the view that we are now dealing with a new situation."
The government did not give details of the plot but a German media report said security officials suspected a number of al Qaeda operatives were planning attacks in Britain and Germany.
De Maiziere said he had instructed German police to step up security measures around the country at potential targets like airports and railway stations, noting that the findings showed Islamic militants were still planning attacks against Germany.
"Citizens will be able to see such police measures. In addition to this, there are a lot of measures that they won't be able to see," he said. "This will apply until further notice."
Germany, which received a tip-off from a foreign intelligence service about the suspected November plot, was working closely with its international partners, de Maiziere said, adding that border controls could be stepped up.
De Maiziere said the situation was comparable to the security alarm surrounding Germany's federal elections in 2009, but he declined to give precise details of the threats.
"You will understand that in the interests of carrying out a successful investigation it wouldn't be clever to make these investigations so public," he said.
Spared so far
German daily Der Tagesspiegel reported after the news conference that security officials believed between two and four al Qaeda militants were about to carry out attacks in Britain and Germany, and were expected to arrive in Germany on November 22.
The attacks could target crowded places like Christmas markets, and it was believed that Pakistan-based militant Ilyas Kahsmiri had masterminded the plan, the report said.
De Maiziere, a senior figure in Merkel's centre-right government, had initially played down the threat last month when the United States and Britain issued warnings that Germany and France could be targeted by al Qaeda and allied militants.
But earlier this month he said there were "serious indications" of a threat to Europe and America.
Germany has long viewed itself as a potential target because it has nearly 5,000 military personnel stationed in Afghanistan, the third largest contingent of the 150,000-strong international force fighting the Taliban-led insurgency.
The country has been largely spared attacks by Islamic militants, although the leaders of the 9/11 attacks on the United States in 2001 were students based in Germany.
De Maiziere said Germany and its allies would do "everything in its power" to keep the public safe from attack.
"We won't be intimidated by international terror, neither in our way of life, nor our culture or freedom," he added.