At least 49 people are dead and more than 40 others seriously injured in New Zealand after at least one gunman opened fire at two mosques.

Authorities have charged one person and detained four others in the attack. They also defused explosive devices after the gunman published live footage of the shooting and published a "manifesto" calling immigrants "invaders."

CNN reports that the attacks took place at two mosques in downtown Christchurch. The majority of the deaths took place at the Masjid Al Noor mosque, where an estimated 41 people are thought to have been killed.

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New Zealand Prime Minister Jacinda Ardern called it “one of New Zealand's darkest days,” according to The Associated Press, adding that it was “an extraordinary and unprecedented act of violence."

“It is clear that this can now only be described as a terrorist attack,” Ardern said.

Ardern raised the national security threat level to the second-highest level. The country's police commissioner, Mike Bush, urged the country's Muslim worshippers to avoid mosques or other places of worship on Friday.

Bush asked "anyone who was thinking of going to a mosque anywhere in New Zealand today not to go. To close your doors until you hear from us again," according to CNN.

The people detained have not been identified but authorities said none had been on any watch list.

One man who claimed to have perpetrated the attack left a more than 70-page anti-immigrant manifesto. Videos and photos on his social media page that appeared to have been taken during the attack were shared on social media before being taken down.

In the online manifesto, the suspect declared himself to be a 28-year-old Australian radicalized online by right-wing influences amid pages of rhetoric against Muslims and other minorities.

Ardern confirmed that one of the attackers detained Friday was Australian, while authorities told CNN that they "will not be discussing the offenders' possible motivations or the causes of this incident" at the present time.

A spokeswoman for Facebook confirmed to the news network that videos appearing to show the attack were deleted from the platform once moderators became aware of their existence.

"New Zealand Police alerted us to a video on Facebook shortly after the livestream commenced and we removed both the shooter's Facebook account and the video. We're also removing any praise or support for the crime and the shooter or shooters as soon as we're aware," she told CNN.

—Updated at 7:33 a.m.