A Norwegian man accused of shooting dead two people on the sidelines of Oslo's 2022 Pride festival pleaded not guilty to committing a "terrorist act", as his trial opened Tuesday.

Zaniar Matapour, a 44-year-old Norwegian of Iranian origin, is accused of opening fire outside two bars in central Oslo, including the gay club London Pub, on the night of June 25 just hours before the Oslo Pride Parade was to be held.

Two men, aged 54 and 60, were killed and nine others were wounded.

Matapour was apprehended by passersby as he tried to flee the scene.

With a bushy grey beard and a traditional Muslim shashiya hat, he appeared in court on Tuesday charged with terrorism for the murders and attempted murders, which the prosecution said were "aimed at spreading fear among the public".

"The terrorist act is serious as it led to the loss of several lives and put other lives in danger," prosecutor Sturla Henriksbo said Tuesday as he read out the charge sheet in the Oslo district court.

Asked if he accepted the charges against him, Matapour replied that he did not.

He also protested against the fact that his trial was being held during the Muslim holy month of Ramadan.

While in custody, he has consistently refused to answer investigators' questions and never provided any motive for the shooting.

One of the Norwegian police's main theories is that he deliberately targeted the gay community because of his extremist religious views.

On an audio recording played by the prosecution, Matapour could be heard swearing allegiance to the Islamic State group just before the shooting.

Experts have been at odds over the suspect's mental health.

In August, two psychiatrists who examined Matapour found he was mentally stable at the time and therefore legally responsible for his actions, but a third expert came to the opposite conclusion.

If he were found legally responsible, he faces up to 30 years in prison, a sentence that can be extended if he were still considered a threat.

If found guilty but not legally responsible, he would be ordered to serve his sentence in a secure psychiatric facility.

The trial, which is scheduled to last until May 16, could also shed new light on the Norwegian domestic intelligence service's shortcomings, after a report concluded it could have prevented the shooting.