The police released a video and photo of the alleged bomber on Friday. The images show a slender person in a blue jacket wearing a motorcycle helmet. In the photo, which does not offer a clear view of the suspect's face, he is seen carrying a backpack and smoking a cigarette.

On Monday, two bombs detonated within half an hour in front of the door of a mosque and on the terrace of the International Congress Center. No one was injured in the blasts.

Unity Day celebrations in Dresden

Extra police have been stationed in and around Dresden and security will be boosted ahead of the Day of German Unity on Monday, with celebrations starting on Saturday. German President Joachim Gauck and Chancellor Angela Merkel are expected at a ceremony held at the International Congress Center, where the second bomb went off.

The city center is already completely closed off to traffic. Some 2,600 uniformed and plainclothes police officers are patrolling the streets. Left-wing and right-wing groups have announced demonstrations and protests on Sunday and Monday.

Fake letter claiming responsibility

A letter claiming responsibility for the bombs was published on a website of the far-left "Antifa" movement on Wednesday. The anti-fascist group denied any links with the explosions and experts quickly identified the letter as fake.

Although no other group has claimed responsibility, the police suspect a xenophobic motive behind the crimes.

Dresden has come under criticism for far-right politics and xenophobia in recent years, beginning with the anti-immigrant and anti-Islam PEGIDA movement founded in the city in October 2014. Monday's bombings came just after PEGIDA staged its latest march through the city.

The state of Saxony has also been the scene of several violent clashes outside refugee shelters.

kw/sms (dpa, afp)