German Foreign Minister Sigmar Gabriel made an unusually strong attack on Chancellor Angela Merkel on Tuesday as politicians blamed each other for the violent protests that overshadowed the G20 summit in Hamburg at the weekend.

The comments come as tension mounts in Germany's ruling coalition between Merkel's Christian Democrats (CDU) and Gabriel's Social Democrats (SPD) just over two months before a national election on Sept. 24.

Some commentators have criticised Merkel's choice of her birthplace, Hamburg, a seaport with a strong radical leftist tradition, to host the meeting. They say her desire to show her commitment to freedom of speech backfired.

On Monday, members of Merkel's conservatives sought to turn the blame back onto Germany's left-wing parties. Some suggested that Hamburg's SPD mayor, Olaf Scholz, should resign for failing to stem the violence.

"Olaf Scholz is not responsible for the staging of this summit," Gabriel told the Funke newspaper group in an interview for publication on Wednesday. "Whoever wants his resignation ... must also demand the resignation of Angela Merkel.

"She carries the political responsibility for the staging and direction of the G20 summit in Hamburg, which had a secret goal: the self-promotion of the CDU leader shortly before the election," he added.

Gabriel's comments drew a sharp rebuke from Andreas Scheuer, general secretary of the Christian Social Union, sister party to the CDU. "Gabriel has blown a fuse," he told the newspaper Bild.

Merkel was more relaxed about Gabriel's remarks, saying she was pleased that the foreign minister had accompanied her to the summit and contributed to its success, adding she was sure the coalition with the SPD would hold until the election.

Speaking at an event in the western city of Essen, Merkel said the G20 had to take place in a large urban area due to the size of the delegations. "A country like Germany must be in the position to hold such summits," she said.

About 20,000 police struggled to contain several hundred demonstrators who torched cars, looted shops and hurled Molotov cocktails and stones during the July 7-8 summit. Almost 500 officers were injured.

In a poll published earlier on Tuesday and conducted before the G20, Merkel's CDU was leading at 39 percent, with the SPD down a point to 22 percent.

Gabriel said the G20 summit had been a "total failure" with respect to the big topics affecting humanity - war, civil war, flight, hunger and poverty.

"But unfortunately today we have to be satisfied if the 20 heads of state of the rich countries can just meet and it doesn't get even worse. So much for a success," he said.