Police in Berlin have arrested a 24-year-old German of Lebanese origin and a 28-year-old Palestinian from the Gaza Strip two men on suspicion of acquiring chemicals for a bomb attack.
Officers raided their homes as well as a mosque in the working class neighbourhood of Wedding on Thursday, a Berlin police spokesman said.
Thomas Neuendorf said authorities had launched the investigation, reportedly codenamed "Rainshower", several months ago but did not provide further details.
He also declined to comment on a possible link to the September 11 anniversary or the late September visit of Pope Benedict XVI to Germany, which will start in Berlin.
"There are no indications of specific targets of an attack," said the spokesman. "The men were still in the planning process."
The news website Spiegel Online quoted police as saying the suspects did not appear to belong to a known terrorism organisation or have links to extremist groups abroad.
Authorities say the men had acquired several coolants and an acid normally used in farming with the aim of building an explosive, the daily Berliner Morgenpost reported online.
Neuendorf said the probe began when the firms where the chemicals were ordered reported the suspicious purchases to police and that the men had not yet started building a bomb.
A spokeswoman for the public prosecutor's office said it had launched a probe against the men on "suspicion of preparing a major violent crime against the state".
The suspects regularly attended the mosque in Wedding and occasionally spent the night there, the Morgenpost said.
Al Jazeera's Nick Spicer, reporting from Berlin, said: "Over 200 police officers were involved in the raid at the mosque."
Officers were out in force at the mosque, where they had closed off the street in front with police tape.
"Witnesses told me the police came in the eraly morning with battering rams, many are still in shock," said journalist.
According to the German interior minister, Hans-Peter Friedrich, up to 1,000 Islamic extremists are believed to be living in Germany who could be described as potential terrorists.
Angela Merkel, the German chancellor, said on Saturday in her weekly video podcast that the fight against terrorism still had to be won a decade after the September 11 attacks, as she defended Germany's unpopular deployment in Afghanistan.
"Osama bin Laden is dead, but terrorism has not yet been defeated," she said.
There have recently been a number of high-profile cases involving men of Muslim origin living in Germany accused of terrorism.
In April, police detained three alleged al-Qaeda members accused of plotting attacks in Germany.
The three were Germans of Moroccan origin from the western state of North Rhine-Westphalia and were reportedly caught with large amounts of explosives.
Al 21-year-old Kosovan went on trial last week for killing two US airmen who were heading to Afghanistan from the western city of Frankfurt in March, in what has been called the first jihadist attack on German soil.