COPENHAGEN -- Danish prosecutors urged a judge on Saturday to hold in pre-trial detention the owner of an amateur-built submarine, suspected of being responsible for the disappearance of a Swedish woman who had been onboard the ship that later sank.

Peter Madsen was arrested Friday on preliminary manslaughter charges after his 40-ton, nearly 18-meter-long (60-foot-long) ship, named the UC3 Nautilus, sank off Denmark's eastern coast. He has denied responsibility for the fate of 30-year-old Kim Wall, saying she had disembarked earlier.

"Peter Madsen faces preliminary charges (of manslaughter) for having killed in an unknown way and in an unknown place Kim Isabell Frerika Wall of Sweden sometime after Thursday 5 p.m.," Prosecutor Louise Pedersen told a packed courtroom, where proceedings began in a larger hall after delays due in part to overcrowding in another location.

The woman's boyfriend alerted authorities that the sub had not returned, prompting a major search involving two helicopters, three ships and several private boats. The Navy said that the sub had been spotted sailing but then sank shortly afterward.

Madsen in the courtroom smiling, chatting with his defence lawyer. "I would very much like to express myself," he said after the preliminary charges were read. The court was then closed for deliberation as judge Kari Soerensen deliberated her decision.

Kristian Isbak, who had responded to the Navy's call to help locate the ship, sailed out immediately Friday and saw Madsen standing wearing his trademark military fatigues in the submarine's tower while it was still afloat.

"He then climbed down inside the submarine and there was then some kind of air flow coming up and the submarine started to sink," Isbak told The Associated Press. "(He) came up again and stayed in the tower until water came into it" before swimming to a nearby boat as the submarine sank, he added.

Madsen told authorities he had dropped the woman off on an island in Copenhagen's harbour a few hours into their Thursday night trip.

"It is with great dismay that we received the news that Kim went missing during an assignment in Denmark," the family said in statement emailed to The Associated Press.

The 30-year-old Sweden-born freelance journalist had studied at the Sorbonne university in Paris, the London School of Economics and at Columbia University in New York where she graduated with a master's degree in journalism in 2013.

She lives in New York and Peking, the family said, and has written for the New York Times, The Guardian, the South China Morning Post and Vice Magazine, among others.

A salvage vessel, the Vina, was Saturday working on raising the submarine, which was seven meters (23 feet) under water off Copenhagen's south island of Dragoer.

In theory, the vessel can dive up to 470 metres (1,550 feet) but has rarely gone deeper than 40 metres (132 feet), according to its Web site.

If tried and found guilty, Madsen would face between five years and life in prison.