A gunman armed with multiple weapons opened fire in a mosque near Oslo on Saturday, injuring one person before being overpowered by an elderly worshipper and arrested, Norwegian police and witnesses said.

Hours after the attack, the body of a young woman related to the suspect was found in a home in the suburb of Baerum where the shooting took place earlier in the day, police said Saturday evening.

Investigators are treating her death as suspicious and have opened a murder probe.

The head of the mosque described the assailant as a young white man dressed in black and said he was wearing a helmet and bulletproof vest.

He said only three people had been inside the al-Noor Islamic center at the time of the attack.

Police were alerted to the shooting shortly after 4:00 pm.

Officers first reported that a victim had been shot, but later clarified one person had sustained "minor injuries" and that it was unclear if they were gunshot wounds.

Police said the suspect appeared to have acted on his own.

"It is a Norwegian young man, with a Norwegian background. He lives in the vicinity," Oslo police spokesman Rune Skjold had told a press conference earlier Saturday. 

Skjold added that the suspect had been known to police before the incident but could not be described as someone with a "criminal background."

The man - who was in his early twenties - was taken into custody, police said in a press release carried by Norwegian media, adding that he would be interrogated "on the events at the mosque and the homicide."

Norway was the scene of one of the worst-ever attacks by a right-wing extremist in July 2011, when 77 people were killed by Anders Behring Breivik. 

"One of our members has been shot by a white man with a helmet and uniform," Irfan Mushtaq, head of the mosque, told local media.

Mushtaq said that the man had carried multiple weapons, but that he had been subdued by a member of the mosque.