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A Danish inventor googled a video of a woman having her head slowly cut off before murdering journalist Kim Wall and dismembering her body, a court has head.

Peter Madsen, 47, went on trial on Thursday accused of tying up and torturing the Swedish reporter before he either cut her throat or strangled her during a trip on his private submarine in August last year.

He is accused of torturing and murdering Wall and dismembering and disposing of her body during a submarine trip last summer.

The inventor, who denies the murder charges but admitted to cutting up her body, had initially maintained that Ms Wall disembarked from the submarine several hours into their trip.

As court proceedings began, the prosecution said Madsen’s phone and computer records showed he had searched for violence towards women.

Copenhagen City Court heard he had googled a video of a woman having her head slowly cut off and also searched the words "beheading" and "girl".

The trial opened with prosecutor Jakob Buch-Jepsen describing in detail how Ms Wall’s body parts were found on the ocean bed.

Ms Wall's torso was found on a southern Copenhagen shoreline in late August. Her head, legs and clothes were discovered in bags at sea in October and November, along with heavy metal objects designed to take them to the ocean floor.

Madsen, wearing glasses and a dark shirt, listened quietly with his fists closed, the Associated Press reports.

The prosecution claims Ms Wall's murder was premeditated because Madsen brought along tools he normally didn't take when sailing.

The 33-tonne, nearly 18-metre-long submarine sank south of Copenhagen shortly after being spotted afloat. Madsen reported "man overboard" over the radio and was then picked up alone.

Mr Buch-Jepsen said that after Madsen was arrested on land, forensic experts found dried blood on Madsen's nose - "blood that eventually was proven to belong to Kim Wall”.

Questioned during the first day of his trial on Thursday, Madsen repeated his earlier claims that Ms Wall died accidentally and said he had offered shifting explanations for her death because "I didn't wish to share with the whole world how Kim Wall died."

Madsen initially told authorities he had dropped Wall off on an Copenhagen island several hours into their submarine trip in August. Then he said that Wall died accidentally inside the submarine while he was on deck during the excursion - a claim he repeated Thursday. He did not elaborate.

He added that he was "in total denial of what had happened" when he told officials over the radio that he had let Swedish journalist Kim Wall disembark on a Copenhagen island.

His defence lawyer, Betina Hald Engmark, urged the court to listen to the facts presented during his trial, not those presented in the media.

"What has been my client's intent? Did he intend to kill Kim Wall?" she asked. "We do not have a cause of death, making it pretty muddy."

Mr Buch-Jepsen said a psychiatric report of the accused has concluded that he is an intelligent man "with psychopathic tendencies" who has "no empathy or feelings of guilt."

Ms Wall studied at Paris' Sorbonne University, the London School of Economics and Columbia University in New York, from where she graduated with a master's degree in journalism in 2013.

She wrote for The New York Times, The Guardian and other publications, reporting on topics such as tourism in post-earthquake Haiti and nuclear testing in the Marshall Islands.

Madsen was a co-founder of Copenhagen Suborbitals, a private aerospace consortium to develop and construct manned spacecraft.

In an interview with Danish weekly Soendagsavisen in 2014, Madsen said he one day "hoped to have a criminal career," adding he didn't want to rob a bank because "no one must be hurt."

On the evening that he contacted Ms Wall, Madsen also texted his associate Steen Lorck to call off a planned trip the following day in the submarine that first launched in 2008.

After Ms Wall left to meet Madsen, her boyfriend received several text messages from her. He started worrying when the messages stopped coming and eventually alerted authorities, who launched a search for the submarine, which didn't have a satellite tracking system.

Investigators found dried blood inside the submarine, and divers eventually found Ms Wall's body parts in plastic bags held down on the Baltic Sea bed by metal pieces. Her torso had been stabbed multiple times.

Police believe Madsen sank the submarine on purpose, and found videos of women being tortured and killed on his personal computer in his hangar. He did not make the videos himself, investigators said.

The trial at Copenhagen's City Court ends April 25. He has opted to have a trial with two judges and two jurors.