Germany promised to protect its synagogues after scattered demonstrations over the Israel-Palestine conflict in which protesters burnt Israeli flags outside Jewish places of worship.
Police in one German city said they stopped protesters from marching on a synagogue.
Israeli flags were burnt outside synagogues in Bonn and Muenster during rallies linked to the violence in the Middle East.
The Central Council of Jews in Germany, which called for increased security at synagogues, said online hatred was targeting not just Israel, but Jewish people in general.
“We expect solidarity with Israel and the Jewish community from German citizens,” it said.
Chancellor Angela Merkel's spokesman said on Friday that attacks on synagogues would not be about criticising Israel's policies "but about aggression and hate towards a religion and the people who belong to it".
"Our democracy will not tolerate anti-Semitic demonstrations," he said.
Armin Laschet, the leader of Germany's conservatives and a contender to replace Ms Merkel as chancellor after September's election, said: "We do not tolerate anti-Jewish polemics, violence, or anti-Semitism. Not in our streets, not in our school yards, not on the internet."
Germany's head of state, Frank-Walter Steinmeier, said people who burnt Israeli flags were abusing their freedom to demonstrate and committing crimes.
"Nothing justifies threats against Jews in Germany or attacks on synagogues in German towns," he told German newspaper Bild.
Rallies across Germany as conflict escalates
Police in Gelsenkirchen said they stopped a march of about 180 people that was heading for a synagogue and where anti-Israel slogans were shouted.
Sixteen people were arrested after the incidents in Bonn and Muenster, when Israeli flags were set on fire late on Tuesday.
In Hannover, police said they broke up a protest of about 550 people and prevented two protesters from burning an Israeli flag.
Another protest in Bremen, where about 1,500 people called for freedom for Palestine, proceeded without incident on Thursday.
In Berlin, a pro-Israel demonstration took place in front of the city's Brandenburg Gate on Wednesday.
German Foreign Minister Heiko Maas said Jewish people should not be held responsible for events in the Middle East.
About 200,000 Jews live in Germany. The community grew after German reunification, with the arrival of Jewish people from the former Soviet Union.
Anti-Semitic crime in Germany increased steadily in recent years, with more than 2,000 offences registered in 2019, according to government figures.
In December, the assailant behind a deadly far-right attack outside a synagogue in 2019 was handed a life sentence.
The gunman failed to storm a synagogue in what could have become Germany's worst anti-Semitic atrocity since the Second World War. He shot dead a female passer-by and a man at a kebab shop.
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