The man, identified as 21-year old Philip Manshaus, appeared in the Oslo court with two black eyes and scrapes and bruises on his face, neck and hands, probably obtained when he was overpowered at the mosque.
Police say he has "extreme right views" and "xenophobic positions" and that he had filmed the mosque attack with a GoPro camera mounted on a helmet.
The Norway incident comes amid a rise in white supremacy attacks around the world.
Manshaus is formally suspected of murdering his 17-year-old stepsister, and of a "terrorist act" at the Al-Noor mosque on Saturday, allegations he has rejected.
In Norway, being formally named as a suspect is a step prior to indictment.
Manshaus entered the courtroom smiling to cameras, wearing dark clothes and his hair cut short.
He asked to be released, his lawyer Unni Fries said after the hearing.
"He rejects the allegations and exercises his right to not explain himself," she said.
The court cited "reasonable grounds" to suspect Manshaus had committed the criminal acts and remanded him in custody for four weeks, as police had requested.
Police official Pal-Fredrik Hjort Kraby told a press conference that Manshaus had worn a helmet equipped with a camera during the attack "which was filming and has provided us with important evidence."
The court said he will be kept in "complete isolation" for the first two weeks.
'Pretty vague' past tip
Manshaus is accused of entering the mosque in the affluent Oslo suburb of Baerum armed with at least two weapons and opening fire before he was overpowered by a 65-year-old man who suffered minor injuries. Just three worshippers were in the mosque at the time.
Hours after the attack, the body of a young woman was found in a home in Baerum and police on Monday identified her as Johanne Zhangjia Ihle-Hansen, Manshaus' 17-year-old stepsister.
Local media said she was of Chinese origin and had been adopted by the companion of Manshaus' father.
On Monday, Norway's domestic intelligence service PST said it had received a tip "about a year ago" about Manshaus, but that they chose not to act on it.
"The tip was pretty vague and was not indicative of any imminent terrorism plot," PST chief Hans Sverre Sjovold told reporters.
PST, which said it receives a large number of similar tips, has not raised the threat alert level in Norway from the current low level.
'Race war' post
Shortly before the mosque attack, a person identifying himself as Philip Manshaus had posted a message on the EndChan forum calling for a "race war" to be taken from the internet into real life ("irl").
The author said he was selected by "saint tarrant", an apparent reference to New Zealand mosque attack suspect Brenton Tarrant, accused of killing 51 people in attacks on two mosques in March.
EndChan confirmed on Twitter that a man "claiming to be the Oslo shooter" had posted material on the forum.
The post ended with the words "Valhalla awaits," a mythological Norse reference to the afterlife for those who have died in battle.
The shooting follows a rise in white supremacist attacks, including the recent El Paso massacre in the United States.
The Oslo attack took place on the eve of Eid Al-Adha, an Islamic celebration that marks the end of the Hajj pilgrimage, and it stoked fears among Norway's Muslims.
In Norwegian media, neighbours and acquaintances described the suspect as a previously happy and well-adjusted person whose behaviour changed during the past year.
According to public broadcaster NRK, he reportedly became "very religious" and adopted increasingly extreme rightwing views.
Norway witnessed one of the worst-ever attacks by a rightwing extremist in July 2011, when Anders Behring Breivik, who said he feared a "Muslim invasion", killed 77 people in a truck bomb blast near government offices in Oslo and a shooting spree at a Labour Party youth camp on the island of Utoya.