The man suspected of a shooting at a mosque in Norway may also have killed a relative before launching the attack, local police have said.

Key points: The suspect is thought to have praised Christchurch attacker online prior to the incident at a mosque on the outskirts of Oslo

The suspect is thought to have praised Christchurch attacker online prior to the incident at a mosque on the outskirts of Oslo A 75-year-old man reportedly overpowered the suspect, preventing deaths

A 75-year-old man reportedly overpowered the suspect, preventing deaths Norway is the site of one of Europe's worst acts of right-wing terrorism in 2011, which killed 77 people

"A young woman was found dead at the suspect's address," Assistant Chief of police Rune Skjold told a news conference, adding that the man was suspected of murder.

Police earlier said a "young white man" had been apprehended following an armed attack at the al-Noor Islamic Centre near the country's capital, adding that members of the congregation had overpowered the gunman and stopped the shooting.

An older man sustained light injuries in the attack but it was too early to say if the wounds were caused by the gunshots or sustained during the attempt to restrain the gunman, according to police.

An initial statement had said one person was shot in the attack.

The 20-year-old suspected attacker appeared to have acted alone, the police added.

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

"The man carried two shotgun-like weapons and a pistol. He broke through a glass door and fired shots," mosque director Irfan Mushtaq told TV2.

The shooter, who wore body armour and a helmet, was overpowered by members of the mosque before police arrived, Mr Mushtaq added.

Police say they will dispatch more officers to protect those celebrating Eid. ( AP: Terje Pedersen )

He said the man who overpowered the shooter was 75-years-old and had been reading the Koran after a prayer session.

"Then I see that there are cartridges scattered and blood on the carpets, and I see one of our members is sitting on the perpetrator, covered in blood," Mr Mushtaq told Norwegian newspaper VG.

According to Mr Mushtaq, the mosque had not received any threats ahead of the shooting.

The attack took place on the eve of the Muslim celebration of Eid al-Adha, marking the end of the Muslim pilgrimage Hajj. Police said they would be sending out more officers so that those celebrating would "be as safe as possible".

Suspect allegedly posted online before attack

A police robot was also dispatched to the scene of the attempted shooting. ( AP via NTB scanpix: Terje Pedersen )

Only three people were present in the mosque at the time of the attack, preparing for Sunday's celebration of Eid, which up to 1,000 people had been expected to attend, mosque spokesman Waheed Ahmed told Reuters.

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Olav Fykse Tveit, general secretary of the World Council of Churches, tweeted that "nobody should pray in fear of attacks like this."

"People of prayer and faith must stand together in times like these."

The mosque earlier this year implemented extra security measures following the massacre of more than 50 people at two New Zealand mosques by a right-wing extremist.

Norwegian police said they were aware that Saturday's suspect had been active online before the shooting.

Broadcaster TV2 reported they had learned the identity of the man and located a post to an online forum from someone using the same name, posted only hours before the attack.

The post seemingly praised the New Zealand attacker and ended with the words "Valhall[a] awaits".

Valhalla refers to heaven where those who die in combat die, as conceived of in Norse mythology — a term which has been used by white supremacist extremists.

There has been a recent spate of white nationalist attacks in the West, including in the United States and the March shootings at two mosques in Christchurch.

The al-Noor Islamic centre in Norway shares its name with the worst affected mosque in the New Zealand attacks.

Local Norwegian paper Budstikka said it had contacted the mosque in March after the Christchurch massacre and that officials there had said security would be tightened.

The suspect in the Christchurch killings wrote a hate-filled manifesto in which he said he was influenced by far-right ideologues including the Norwegian mass murderer Breivik.

Breivik detonated a bomb in Oslo that killed eight people and then opened fire on a gathering of the Labour Party's youth wing on the island of Utoya, killing another 69 people, most of them teenagers.

ABC/Wires