(CNN) At least 49 people were killed and 20 seriously injured in mass shootings at two mosques in the New Zealand city of Christchurch Friday, in a carefully planned and unprecedented atrocity that shocked the usually peaceful nation.

The attack was unleashed at lunchtime local time Friday, when mosques were full of worshippers. Footage of the massacre was streamed live online, and a rambling manifesto laced with white supremacist references was published just before the shootings unfolded.

Prime Minister Jacinda Ardern described the horror as a terrorist attack, saying it was perpetrated by suspects with "extremist views" that had no place in her country or the wider world. It was one of the New Zealand's "darkest days," she said in a press conference Friday.

Authorities said that every law enforcement resource in the country was mobilized after the attack.

Three people were arrested in connection with the shootings. A 28-year-old man was charged with murder and will appear in court Saturday morning local time. Two others were arrested on suspicion of possession of firearms. Police were investigating their ties to the incident, New Zealand Police Commissioner Mike Bush said.

Australian Prime Minister Scott Morrison said that at least one of those arrested is Australian. The atrocity was the work of an "extremist right-wing, violent terrorist," he said.

Police were not searching for any other suspects in connection with the attack but stressed the investigation remained fluid. None of those arrested in connection with the attacks had been on any security watch lists prior to the attack.

A total of 48 people, including young children with gunshot wounds, were taken to hospital.

Attack apparently broadcast live on social media

Authorities declined to discuss the potential motives behind the attack. But in a social media post just before the shooting began, an account believed to be linked to the gunman posted a link to an 87-page manifesto that was filled with anti-immigrant, anti-Muslim ideas and explanations for an attack. The manifesto was not signed.

Police said they were aware of a video shared online and broadcast live during the attack, which apparently shows a gunman walking into a mosque and opening fire. "We would strongly urge that the (video) link not be shared. We are working to have any footage removed," New Zealand police said.

The brazen nature of of the broadcast, and the tech companies' failure to prevent its proliferation online, raised profound questions about the nature of internet radicalization. In New Zealand, commentators expressed concern that the horror would sow deep divisions in a society that has largely avoided the polarizations that have spread elsewhere.

A police officer secures the area in front of the Masjid al Noor mosque.

In a statement, a spokesperson for Facebook New Zealand, Mia Garlick, said the footage was quickly taken down. "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 said.

CNN has not been able to independently confirm any information about any of the attackers or the the alleged video at this stage.

A spokesperson for Google and YouTube called the shooting a "terrible tragedy." A statement said: "Shocking, violent and graphic content has no place on our platforms, and is removed as soon as we become aware of it. As with any major tragedy, we will work cooperatively with the authorities."

Twitter removed an account it believed was linked to the main suspect and was working to keep the video of the incident off its platform, a spokesperson said. "We are deeply saddened by the shootings in Christchurch today," Twitter said in a statement. "Twitter has rigorous processes and a dedicated team in place for managing exigent and emergency situations such as this. We also cooperate with law enforcement to facilitate their investigations as required."

IEDs found in vehicle

Armed police were deployed after first receiving reports of the shootings at 1:40 p.m. local time, when mosques were packed for Friday prayers. Police said 41 people were killed at the al Noor mosque on Deans Avenue. Seven died at the Linwood mosque on Linwood Avenue, and one died in the hospital from injuries.

Two improvised explosive devices were found in a vehicle linked to the attack. A number of weapons were also recovered at both locations.

The area was placed on lockdown, and police urged Christchurch residents to stay indoors and monitor the police website and social media. Worshipers were told to stay away from all mosques in New Zealand.

Later in the day, authorities evacuated properties close to a "location of interest" in the southern city of Dunedin, some 225 miles from Christchurch.

Gunman opened fire 'for 10 to 15 minutes'

One worshiper, Mohan Ibn Ibrahim, said he was inside one of the mosques when the shooting began and that he heard the gunman "continuously shooting for 10 to 15 minutes."

"It's a big mosque and there were more than 200 people inside. The gunmen came from the back side. Gunshots went on for a long time. We had to jump the wall to escape. I saw lots of broken glass and bricks on the backside of the mosque," he said.

People wait outside a mosque in central Christchurch, New Zealand, Friday, March 15, 2019.

"I came to the street I saw one person got shot on his chest," he said, adding that the ambulance and police then arrived on the scene. He said that he had a friend in another mosque in the area who told him a gunman had opened fire there as well and five people were dead.

"I could not contact two of my friends who are in the mosque as well," he said.

Another witness, who did not wish to be named, said he was driving by the scene and saw a man with a "with his 3- or 4-year-old daughter" who had been shot in the back. "He was screaming like get her to the hospital and the ambulance couldn't come in until it was secured so I just got my truck and loaded up him, and his daughter, and this other guy had been shot in the leg, and took them to the hospital," he said.

One man outside the mosque said that he prayed that the gunman would "run out of bullets."

"I was thinking that he must run out bullets you know, so what I did was basically waiting and praying to God, oh God please let this guy run out of bullets," he said. He said a man told him to remain still and then the gunman shot the man "straight in the chest."

Prime Minister Jacinda Ardern speaks in Wellington, New Zealand.

In a press conference, Prime Minister Ardern described the attack as "one of New Zealand's darkest days."

"What has happened here is an extraordinary and unprecedented act of violence," she said, adding that the attackers have "no place in New Zealand."

"For now my thoughts and I'm sure the thoughts of all New Zealanders are with those who are being affected and with the families," she said.

World leaders react

Australian Prime Minister Morrison said he has asked for flags to be flown at half-staff out of respect for those killed in the attack. "Australians stand with all New Zealanders today during this dark time where hate and violence has stolen their peace and innocence. Kia kaha (stay strong)," Morrison tweeted earlier.

US President Donald Trump said in a tweet: "My warmest sympathy and best wishes goes out to the people of New Zealand after the horrible massacre in the Mosques. 49 innocent people have so senselessly died, with so many more seriously injured. The U.S. stands by New Zealand for anything we can."

My warmest sympathy and best wishes goes out to the people of New Zealand after the horrible massacre in the Mosques. 49 innocent people have so senselessly died, with so many more seriously injured. The U.S. stands by New Zealand for anything we can do. God bless all! — Donald J. Trump (@realDonaldTrump) March 15, 2019

In a statement, the White House said: "The United States strongly condemns the attack in Christchurch. Our thoughts and prayers are with the victims and their families. We stand in solidarity with the people of New Zealand and their government against this vicious act of hate."

Yousef bin Ahmad Al-Othaimeen, the Secretary General of the Organization of Islamic Cooperation (OIC), condemned the attacks. The OIC represents 57 nations with large or majority-Muslim populations.

"The brutal crime had shocked and hurt the feelings of all Muslims around the world, and served as a further warning on the obvious dangers of hate, intolerance, and Islamophobia," he said. Othaimeen urged the New Zealand authorities to investigate the attack thoroughly.

Sadiq Khan, the mayor of London, tweeted his condolences to the victims of the attack, and confirmed that there will be "highly visible" and armed police around mosques in the UK capital on Friday.

"I want to reassure the Muslim communities in London. I have been in touch with the Met Police. There will be highly visible policing around mosques today, as well as armed response officers, as Londoners go to pray," he said.

UK Prime Minister Theresa May sent her "deepest condolences" in a tweet. "My thoughts are with all of those affected by this sickening act of violence," she wrote.