The Danish inventor accused of torturing and killing Swedish journalist Kim Wall before dismembering her body is an intelligent man “with psychopathic tendencies”, a prosecutor said as his trial opened.

Peter Madsen, 47, was standing trial on Thursday in Copenhagen accused of torturing Wall, 30, before he either cut her throat or strangled her on his submarine in August.

The inventor is charged with murder, dismemberment and indecent handling of a corpse.

He denies murder, but has admitted to dismembering her body before he “buried her at sea”.

Ms Wall, a freelance journalist who wrote for The New York Times, The Guardian and other publications, embarked on Madsen’s submarine on August 10 to interview him.

A court sketch shows Peter Madsen (left) and prosecutor Jakob Buch-Jepsen (right) on the first day of the trial. Photo: AAP

Her remains were found in plastic bags on the Baltic Sea bed weeks later, and her torso was found stabbed multiple times.

Prosecutor Jakob Buch-Jepsen started the 12-day trial by reading out the charges, describing in detail how Ms Wall’s body parts were found on the ocean bed.

He said a psychiatric report has concluded that Mr Madsen has “no empathy or feelings of guilt”.

Giving evidence, Mr Madsen repeated his claim that Ms Wall died accidentally inside the UC3 Nautilus while he was on deck.

He said Ms Wall “had a wonderful evening until it ended in an accident”, but denied that any sexual activity had taken place between them.

Mr Madsen had offered shifting explanations for Ms Wall’s death prior to the trial.

He initially told authorities he had dropped Ms Wall off on an Copenhagen island several hours into their submarine trip.

Then he said that Ms Wall died accidentally inside the submarine when a hatch fell and hit her on the head.

On Thursday, he claimed there was a sudden pressure problem in the submarine.

The turret hatch locked and Mr Madsen says he was unable to let Ms Wall out. When he eventually could enter the hull, he says he found Ms Wall lifeless.

“The atmosphere in the Nautilus did not sustain life at this moment,” he said.

In September last year, Ms Wall’s family shared a tribute video to the late journalist on Facebook, featuring old family footage of her as a young child and quotes from loved ones.

-AAP