Australian prime minister Scott Morrison confirms one of those arrested is Australian.
A suspected gunman broadcast live footage on Facebook of the attack on one mosque in the city of Christchurch, mirroring the carnage played out in video games, after publishing a "manifesto" in which he denounced immigrants.
Ardern also said that Trump conveyed "his absolutely horror and disgust" about the attack.
He fired more shots on the street at no apparent target and said: "Looks like we won't get the bird today boys". The attacks seem to be a carefully planned and the authorities detained four people and defused explosive devices that are said to have belonged to the attackers.
New Zealand Police are dealing with an active shooter incident after a man reportedly burst into a mosque in Christchurch and opened fire. The death toll is now at 49 but may rise. There was a second shooting at the Linwood Masjid Mosque, about two miles east of the Deans Avenue building, that PM Ardern said, killed 10 people.
"Authorities said they had four people in custody - three men and one woman - but later clarified that only three were believed to have been involved in the violence", The Washington Post reported. "The persons who has perpetuated this violence against us. have no place in New Zealand".
Prime Minister Imran Khan condemned the terror attacks in New Zealand on Friday.
Rockets fired from Gaza at Tel Aviv, explosions heard: Israeli military, witnesses
Israeli analysts speculated that Palestinian militants opposed to any deal between Hamas and Israel were behind the launchings. He said that while "rogue factions" were behind recent Gaza "provocations", it "did not exempt Hamas" of responsibility.
People comfort each other after a mass shooting targeting two Christchurch mosques.
"Our hearts are broken over today's awful tragedy in New Zealand", YouTube, which is operated by Google, said in a Twitter posting. He described the incident as a terrorist attack.
"I'm also anxious about New Zealand because NZ is one of the most peaceful countries in the world, very good reputation so that image can be affected by this kind of activity".
Using what may have been a helmet camera, he livestreamed to the world in graphic detail his assault on worshippers at Christchurch's Al Noor Mosque, where at least 41 people were killed.
"Please do not try and come and collect your children until police say it is safe for people to come into the central city", they said. "Many of those affected will be members of our migrant communities New Zealand is their home they are us", Ms Ardern said.
The Prime Minister also offered strong words in defence of a now hurting New Zealand: "This act is not a reflection of who we are as a nation. this isn't who we are".