US secret services keep tabs on around half a billion telephone calls, e-mails and mobile phone text messages in Germany a month, it was reported yesterday.
The claims in German news magazine Der Spiegel cited figures from National Security Agency (NSA) documents.
The metadata - showing when a communication was made and with whom - is saved at the agency's headquarters, it said. The magazine had reported on Saturday that the NSA had bugged European Union offices in Washington and Brussels.
It said the information was based on confidential documents, some of which it had been able to consult via fugitive Edward Snowden.
Yesterday, the leaders of Germany and France joined the European Union in demanding answers from the US over the allegations.
While in Hong Kong, Snowden revealed to the South China Morning Post that the US monitors hundreds of thousands of computers around the world, including Hong Kong and the mainland. The US government has defended its efforts to intercept electronic communications overseas by arguing that this has helped prevent terror attacks at home and abroad.
But the president of the European Parliament, Martin Schulz, said he was "deeply worried and shocked about the allegations".
He said if the allegations were confirmed "it would be an extremely serious matter which will have a severe impact on EU-US relations".
The German Federal Prosecutors' Office said it was still deciding whether a formal investigation was warranted.
"It's beyond our imagination that our friends in the US consider the Europeans as enemies," German Justice Minister Sabine Leutheusser-Schnarrenberger said. In Paris, Foreign Minister Laurent Fabius said he had asked the US for an explanation.
He said: "We expect the American authorities to answer the legitimate concerns raised by these press revelations as quickly as possible." The Der Spiegel report said the information showed that in December, the NSA intercepted metadata from an average of about 15 million phone conversations daily and about 10 million internet connections in Germany. On some days, the figures were much higher.
It gave a comparison for France, where Der Spiegel said the US recorded data from a daily average of around two million connections in the same timeframe, adding that the NSA was more active in Germany than any other EU country.
Der Spiegel also said an internal classification revealed that Germany was categorised as a "third party" partner.
Britain, Australia, Canada and New Zealand, which are second-party partners, are exempt from surveillance, it added.
Reuters, Associated Press, Agence France-Presse