Climate activists gathered outside the Olympic Hall venue in the southern German city as the meeting got under way on Wednesday.

The entrance was briefly blocked, according to the police.

They carried banners condemning Siemens for its partnership with Indian conglomerate Adani, which is building one of the world's largest open-cast mines in Queensland.

Some declared, "We don't need coal," while others accused Siemens of supporting "environmental criminals".

Siemens is to build the Adani mine's rail-signalling infrastructure under a deal worth 18 million euros ($A29.5 million).

Siemens chief executive Joe Kaeser decided to go ahead with the project in January, despite reassessing the contract in light of major protests in Germany and Australia.

On Wednesday, many of the protesters formed a human chain.

Among them were supporters of the German branches of the Fridays for Future and Extinction Rebellion climate movements.

Six protesters from Extinction Rebellion glued themselves to a railing inside the building, and glued their hands together.

Luisa Neubauer, an activist with Fridays for Future in Germany, said the protest was not just about a coal mine but about "fundamental decisions".

The topic dominated speeches at the AGM.

Varsha Yajman, of Australia's branch of Fridays for Future, said it was a "shame" that Siemens signed the contract with Adani while Australia was on fire.

Helena Marschall, also from Germany's Fridays for Future movement, told Kaeser "you are losing your customers - at least the future ones".

Kaeser responded: "We agree on the diagnosis. But so far only we have begun the therapy."

Kaeser appeared frustrated by the criticism, saying it was "grotesque" that the project in Australia had made Siemens a target of environmental protests.

"You cannot win on such issues, because the claim that many have on the other side is a legitimate one," he said.

Australian Associated Press