Three people were killed and five seriously injured in a knife attack in southern Germany on Friday by a 24-year-old Somali immigrant, described by officials who revealed he had been placed under compulsory psychiatric treatment in recent days.

Police stopped and arrested the attacker in the town of Wuerzburg by firing a bullet through his thigh. They said his injuries were not life-threatening. Among the three dead was a young boy and one of his parents, Main Post newspaper reported.

"Three are dead and five very seriously injured," regional Interior Minister Joachim Herrmann said after arriving on the scene. "With the most seriously injured, we are not sure they will survive."

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The man had lived in Wuerzburg since 2015, the year Germany opened its borders to more than a million migrants and refugees fleeing war and poverty. 

"His condition had been noticed in recent months, including violent tendencies, and a few days ago, he was put into compulsory psychiatric treatment," Herrmann said.

Later, he told public television that, according to one witness, the suspect had called "Allahu akbar," an Arabic phrase meaning God is great. "That suggests a possible Islamist motive, and that is also part of the investigation," he added.

Germany's President Frank-Walter Steinmeier pointed to the "extreme brutality" of the crime. "He will be held to account by the rule of law for this inhumane act," Steinmeier told reporters.

"We are mourning in all of Germany today with the relatives of the victims," he said wishing the injured a swift recovery. 

As is standard practice in Germany, police have not released the name of the suspect. Authorities said there was no indication that there were any other attackers and that the situation was now under control.

Videos posted on social media showed a seemingly barefoot man holding a long knife being warded off by other men with chairs until police arrived. Another video appeared to show blood on the ground.

The videos matched the reported location of the attacks on and around Wuerzburg's central Barbarossaplatz, though it was not immediately possible to verify the content.

Praise poured in from political leaders, thanking members of the public for their courage.

"My great respect goes to the brave citizens who quickly intervened," tweeted Armin Laschet, the leader of Merkel's CDU party, sending his condolences to relatives of the victims.