By Ben Westcott, Jenni Marsh, Helen Regan, Meg Wagner, Brian Ries, Veronica Rocha, Aimee Lewis, Rob Picheta and Harmeet Kaur, CNN
Asked by reporters how the attack had occurred, New Zealand Prime Minister Jacinda Ardern said the four people under arrest had not been on any security watch lists prior to the attack.
When pressed on why the intelligence services hadn't been watching the attackers, Ardern said there were questions to be asked.
"I think we should be vigilant against the idea of extreme ideology and violent acts," she said.
New Zealand Prime Minister Jacinda Ardern said the attack appeared to be well planned and said the country had been chosen for its strong multicultural values.
"We were chosen (because) we represent diversity, kindness compassion, a home for those who share our values, refuge for those who need it and those values, I can assure you, will not and cannot be shaken by this attack," she said.
Ardern said while the attackers may have chosen New Zealand, but "we utterly condemn and reject you."
New Zealand Prime Minister Jacinda Ardern has confirmed 40 people have been killed and at least 20 others injured in Friday's mosque attacks in Christchurch.
Australia's Prime Minister Scott Morrison has said at least one of the Christchurch attackers is an Australian born citizen.
Speaking at a press conference in Sydney Friday, Morrison condemned the attackers as "extremist right wing, violent terrorists."
"Australia and New Zealand, we're not just allies, we're not just partners, we're family and as family members with our New Zealand cousins today, we grieve, we are shocked, we're appalled, we're outraged," he said.
Police in Los Angeles, New York and Minneapolis have announced they will be reassessing security at mosques in both cities in the wake of the New Zealand attacks.
“There are no known credible threats to our mosques in Minneapolis and we will be providing additional patrols,” the Minneapolis police force said.
Los Angeles police said their thoughts and prayers were with those in Christchurch after the "horrific" attack and would be providing "extra patrols around mosques."
New Zealand's world famous All Blacks rugby team said in a statement they "stand with" Christchurch and the victims of the mosque attacks.
"Our thoughts and sympathies are with everyone affected by today's tragedy. Stay strong. Kia Kaha," the team said on their official Twitter.
New Zealand boxer and rugby player Sonny Bill Williams posted an emotional video to Twitter, saying he "couldn't put into words what he was feeling right now."
"Everyone that has been killed today in Christchurch, your families ... You guys are all in paradise and I'm just deeply deeply saddened that this would happen in New Zealand," Williams said through tears.
Rosemary Omar has been waiting for four hours close to one of the mosques for word of her son, 24-year-old Tariq.
Rosemary said she dropped her son off at the mosque then drove round the back to find a car parking space when she heard multiple gunshots ring out.
She drove back around the front of the mosque and saw "lots of bodies outside."
"We've just been waiting here since just to see if our son is alright but he's not answering his phone," she said.
Rosemary said police have told her to wait for news of whether her son is alive or dead. "I just feel quite dead to be honest, quite numb. I don't know," she said.
"It just doesn't feel real."
So far, police have arrested four people in connection with the attack on two Christchurch mosques -- three men and one woman.
In a social media post just before the attack, an account that is believed to belong to one of the attackers 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.
TVNZ journalist Anna Burns-Francis told CNN at least one of the shooters is believed to be an Australian national, while a witness from the scene described a man "white, aged in his 30s or 40s and wearing a uniform."
CNN has not been able to independently confirm any information about any of the attackers at this stage.
A spokeswoman for Facebook New Zealand, Mia Garlick, said videos which appeared to show the Christchurch shootings have been taken down.
"New Zealand Police alerted us to a video on Facebook shortly after the livestream commenced and we quickly removed both the shooter’s Facebook and Instagram accounts and the video," the spokeswoman said.
The disturbing video which has not been verified by CNN, appears to show a gunman walking into the mosque and opening fire.
New Zealand police have asked social media users to not share the video which has already been distributed widely online.