German Chancellor Olaf Scholz on Saturday visited the site of a car-ramming attack on a crowded Christmas market that killed five people and injured more than 200 as he called for unity while condemning the "terrible catastrophe."

Police arrested a 50-year-old Saudi doctor of psychiatry at the scene, next to the battered BMW SUV that had ploughed through the festive crowd on Friday night, leaving a trail of carnage and bloodied casualties.

A sombre Scholz, dressed in black, was joined by national and regional politicians in the eastern city of Magdeburg, where they laid flowers outside the main church.

He pledged that Germany would respond "with the full force of the law" to the attack but also called for unity as Germany has been rocked by a heated debate on immigration and security as it heads towards elections in February.

The centre-left chancellor said it was important "that we stick together, that we link arms, that it is not hatred that determines our coexistence but the fact that we are a community that seeks a common future."

He said he was grateful for expressions of "solidarity ... from many, many countries around the world" and added that "it is good to hear that we as Germans are not alone in the face of this terrible catastrophe."

Mourning and bereaved residents had already left candles, flowers and children's toys at the Johanneskirche church, where a memorial service was planned at 7:00 pm (1800 GMT).

As Germany was reeling from the shocking attack, which came eight years after a jihadist strike on a Berlin Christmas market claimed 13 lives, more details emerged about the Saudi man under arrest.

Named by German media as Taleb A, he was a doctor who had lived in Germany since 2006 and held a permanent residence permit, working in a clinic near Magdeburg.

He had long also worked as a rights activist who supported Saudi women and described himself as a "Saudi atheist". He had voiced strongly anti-Islam views, echoing the rhetoric of the far-right, according to his social media posts and past interviews.

As his views expressed online grew more radical, he accused Germany's past governments of a plan to "Islamise Europe" and voiced fears he was being targeted by authorities.

The Bild daily reported that an initial drug test had proved positive, after police officers on Friday used a test kit that can detect narcotics ranging from cannabis to cocaine and methamphetamines. (AFP)