Germany's parliament building has closed its popular cupola to visitors amid heightened security.
The Reichstag's panoramic viewing area, which attracts up to 10,000 people a day, will be shut until further notice.
Other security measures around the building, visited annually by about one million, have also been increased.
Germany's Der Spiegel magazine has reported the discovery of plans by al-Qaeda linked militants to break into the Reichstag and take hostages.
A spokeswoman for the parliament building told the BBC that guided tours for visitors would continue, despite the alleged threat.
Security level raised
The Der Spiegel report says an informer, who lives abroad, contacted German authorities to warn that a six-strong militant cell was planning an attack in February or March. The informer said two members of the cell were already in Berlin.
He said the other four - a German, a Turk, a North African and a man of unspecified origin - were trying to enter Germany.
The magazine said the informer was a militant who wanted to abandon the group. He feared for his life and wanted to return to his family in Germany, it said.
The informer said the Reichstag was only one of the targets of the planned attack.
German security officials have played down the reports in Der Spiegel.
Joerg Ziercke, head of the Federal Crime Office, said: "We have concrete details of suspects, but no concrete details that an attack will be carried out at a specific time and place."
Last Wednesday, Interior Minister Thomas de Maiziere raised Germany's security level, citing increased threats from extremists.
German officials said they had received a tip-off from an unspecified country about a suspected attack planned for the end of November.
Mr De Maiziere said last Wednesday: "There is reason for concern, but no reason for hysteria."
Security was strengthened in airports, train stations and other public places.
The increase in the security level came as German officials said that a suspect package recently intercepted at Windhoek airport in Namibia and about to be loaded on to a Munich-bound flight was a dummy case used to test security.
German and Namibian officials are still trying to determine who placed the case. No explosives were found.