An attack at Berlin's main train station was thwarted on Monday when workers found a hidden device before it could explode, police said.
Earlier Monday, a similar device exploded near a high-speed train line west of Berlin, causing some damage but no casualties, and halting train traffic there for the day.
An unknown leftist group claimed responsibility for the device that exploded, saying it wants to protest Germany's role in the war in Afghanistan.
Police were examining the message's authenticity and trying to determine if the same group was responsible for both attacks.
At the train station near Germany's Chancellery in central Berlin the explosive device discovered by railway employees was set to explode soon, police said. Experts secured seven bottles filled with flammable liquid bundled together and linked to a fuse, Federal Police spokeswoman Nadine Marschner said.
Railway employees alerted authorities at midday after detecting suspicious items at the north end of one of the station's tunnels, 100 yards (meters) from train platforms, she added.
After the earlier attack _ during which an explosion occurred in a utility shaft near a high-speed train line linking Berlin and Hamburg _ police said they obtained a letter in which the leftist group claimed responsibility for the attack, and its authenticity was being examined, Brandenburg Criminal Police spokesman Toralf Reinhardt told German news agency dapd.
In a similar letter posted on a leftist website, a group calling itself Hekla Reception Committee _ Initiative for more Eruptions in Society claimed responsibility, saying it launched the successful attack to protest Germany's roles in the Afghanistan war and as an arms exporter. The group's name is an apparent reference to Iceland's Hekla volcano.
"We dedicate our action to Bradley Manning," the letter said, referring to the U.S. Army private suspected of supplying the many classified documents released by the WikiLeaks website.
Gerd Neubeck, the head of security at German railway operator Deutsche Bahn, denounced the attack, saying: "Our customers shall not be held responsible for the German military's mission in Afghanistan."
Berlin has seen several recent arson attacks that police have blamed on leftist extremists.
One of them in May targeted a utility shaft at an important railway hub in the city's east, causing massive disruption that left thousands of passengers stranded.
Germany is a large contributor to the NATO-led International Security Assistance Force in Afghanistan, with about 5,000 soldiers stationed in the north of the country.