BERLIN (Reuters) - Unidentified attackers set fire to the car of Frauke Petry, the leader of Germany's anti-immigrant Alternative for Germany (AfD) party, police said on Saturday.

"We are currently assuming it was arson," a police spokeswoman in the eastern city of Leipzig said, adding that investigators were still collecting evidence at the scene.

The attack happened late on Friday and there has been no claim of responsibility so far, the police spokeswoman added.

Petry wrote on Twitter: "An arson attack was committed on my car yesterday. Is this what we have come to..."

Last year unknown attackers set fire to the car of AfD deputy leader Beatrix von Storch in Berlin.

The right-wing AfD has gained support as voters become uneasy with Chancellor Angela Merkel's open-door policy toward refugees. Around one million migrants arrived in Germany last year, many fleeing conflicts in the Middle East.

The AfD won a shock 20.8 percent in an election in the eastern state of Mecklenburg-Vorpommern two weeks ago, knocking Merkel's conservatives into third place.

Merkel's party looks set to suffer a second electoral blow on Sunday in a Berlin city election.

(Reporting by Michael Nienaber and Hans-Edzard Busemann; Editing by Gareth Jones)