BERLIN (Reuters) - A double-digit score for the anti-immigrant Alternative for Germany (AfD) in a Berlin city vote on Sunday would be seen around the world as the rebirth of the Nazis, the mayor of the German capital has warned.

The right-wing AfD has gained support as voters become increasingly uneasy with Chancellor Angela Merkel's open-door refugee policy, which saw about one million migrants arrive in Germany last year.

A poll by Forschungsgruppe Wahlen for broadcaster ZDF that was published on Thursday showed the AfD was set to get 14 percent in the weekend vote in Berlin, historically a left-wing stronghold.

"It would be seen around the world as a sign of the return of the right-wing and the Nazis in Germany," Berlin Mayor Michael Mueller, a Social Democrat (SPD), wrote on Facebook on Thursday.

"Berlin is not any old city - Berlin is the city that transformed itself from the capital of Hitler's Nazi Germany into a beacon of freedom, tolerance, diversity and social cohesion," he said.

The center-left SPD runs the city of Berlin in coalition with Merkel's Christian Democrats (CDU).

The AfD won a shock 20.8 percent in an election in the eastern state of Mecklenburg-Vorpommern two weeks ago, meaning the party is now represented in nine of Germany's 16 state assemblies.

When migrants started arriving in large numbers about a year ago, some were met with applause, cheers and gifts, but the mood has since shifted due to concerns about integration and attacks by asylum seekers on civilians this summer.

On Wednesday, locals and asylum seekers clashed in the eastern town of Bautzen. About 80 young people, mainly Germans described by police as being right-wingers, chanted that the town belonged to Germans as 20 asylum seekers stood opposite them. The groups threw bottles and wooden slats at each other.

The Forschungsgruppe Wahlen poll on the Berlin city election showed the SPD on 23 percent, followed by the CDU on 18 percent, the Greens on 15 percent and the far-left Linke on 14.5 percent. The pro-business Free Democrats were on 6.5 percent.

(Reporting by Michelle Martin; Editing by Louise Ireland)