BERLIN (Reuters) - Unknown perpetrators set fire to three police cars on Saturday night in the eastern city of Dresden, where security has been tightened for three days of events to mark 26 years since German reunification, police said on Sunday.

Two improvised bombs exploded in the city last week, one at a mosque and one at an international conference center, and around 2,600 police officers are on duty to safeguard events.

No-one was hurt in Saturday night's blaze, which investigators believe was a politically-motivated act linked to the police operation for the anniversary celebrations.

Dresden was the cradle of the anti-Islam PEGIDA grassroots movement, whose weekly rallies attracted around 20,000 supporters at their height at the start of 2015.

The influx of almost a million migrants last year, mainly Muslims from the Middle East, Africa and Asia, has increased social tensions.

Far-right violence and attacks on migrants have risen dramatically, with riots and arson attacks on refugee shelters in the towns of Heidenau and Freital in the state of Saxony, of which Dresden is the capital.

German Chancellor Angela Merkel, who grew up in Communist East Germany criticized the "shamefully high number" of xenophobic attacks in eastern Germany in an interview published on Saturday.

In her weekly podcast, Merkel also condemned misuse by far-right extremists of the phrase "Wir sind das Volk" (We are the people), a slogan originally used by East Germans in protests leading up to the fall of the Berlin Wall in 1989.

Merkel is due to visit Dresden on Monday along with German President Joachim Gauck, a former pastor in East Germany.

(Reporting by Caroline Copley and Thomas Seythal; Editing by Andrew Bolton)