Islamist terrorists have been planning to attack Berlin's Reichstag building, the home of the German parliament, Spiegel reported on Saturday.
Meanwhile, the head of airport security at Windhoek airport in Namibia was arrested over a security alert triggered by a dummy detonator in luggage bound for Germany earlier this week, the interior ministry said.
The developments came amid a heightened security alert, following an announcement on Wednesday by Interior Minister Thomas de Maiziere that there were "concrete" indications that Islamist militants were plotting attacks in Germany.
Al Qaeda and associated Islamist groups were planning to take hostages and fire at people in the Reichstag, which is also a popular tourist attraction, Spiegel reported in an article to be published Monday.
News of the plot reportedly came from a jehadi who contacted the German authorities, prompting de Maiziere's warning earlier in the week of planned attacks in Germany.
Spiegel reported that the informant repeatedly telephoned police from a foreign location in recent days, asking the authorities to help him return to his family in Germany.
He said the terrorist cell consisted of six people, of whom two had arrived in Berlin weeks ago. Four other militants - including a German, a Turk and a northern African - were still waiting to travel to Germany, Spiegel said.
The attacks were seemingly planned for February or March 2011, and possibly modelled on the November 2008 attacks in Mumbai, in which more than 166 people died when Islamist militants attacked a series of high-profile hotels, a train station and Jewish centre.
On Saturday, undeterred visitors continued to queue up to enter the Reichstag. Security had not been visibly stepped up at the building, which has a permanent police presence.
Chancellor Angela Merkel told citizens not to be intimidated by reported terrorist plots, while also acknowledging that the threat was "real".
"We want to live freely and without fear in Germany - no terrorist threat will prevent us from doing so," Merkel told Bild am Sonntag newspaper.
Federal police chief Matthias Seeger said the danger was greater than it had been ahead of last year's general election or during the World Cup in 2006, in part because the indications had come from "several sources".
Days earlier, US authorities had separately warned Germany of planned attacks, Spiegel wrote.
The Federal Bureau of Investigation (FBI) reportedly mentioned a Shia Indian group called Saif (Sword), which had aligned itself with Al Qaeda, which is Sunni. Two of their men were due to travel from the United Arab Emirates to Germany in coming days.
One mastermind cited by the FBI was Dawood Ibrahim, Spiegel wrote, a man branded by the US as a "global terrorist" and described by the UN as a supporter of terrorism. He is thought to have been instrumental in the 2008 Mumbai attacks.
While police took the tip-offs seriously, German and US intelligence agents reportedly expressed doubts over the likelihood that a Shia group would join forces with Al Qaeda, and the fact that there was no prior evidence of attacks planned from India.
Criminal police (BKA) chief Joerg Ziercke would not comment on the Spiegel report but said it was evident that, "there could be a focus on symbolic objects across Germany".
Aiman Mazyek, the head of Germany's Central Council of Muslims, feared that the current atmosphere could lead to suspicion toward his fellow believers.
"Muslims are doubly scared - because they could become victims of such an attack, and since general suspicions are formed in this way," Mazyek said.
"The current terrorist hysteria is worsening the climate, reducing solidarity with Muslims and strengthening those people who plan to attack them," he added.
On Wednesday, security was increased at airports and train stations across Germany after de Maiziere said there were "concrete" indications that militants were planning attacks in Germany.
The same day, police in Namibia found a suitcase containing a detonator wired to batteries and a clock, amongst luggage bound for Germany - a device that turned out to be a dummy, used to highlight security loopholes.
The head of airport security at Windhoek airport admitted to having placed the suitcase on a luggage conveyor belt, Namibian police chief Sebastian Haitota Ndeitunga told the country's German-language daily Allgemeine Zeitung.
The suspect, a high-ranking officer who had worked in the police for five years, was spotted on video surveillance footage. He was arrested Friday, and initially denied the claims. He is to appear in court Monday.