He appeared in court in white prison clothing, handcuffed and barefoot.
Tarrant did not apply for bail or to have his name suppressed and has been remanded in custody.
At least 49 people were killed and more were injured.
A man in his 20s has been charged with murder and will soon appear in court, according New Zealand's police commissioner.
The attack led to an outpouring of grief and shock that a white-supremacist fanatic could carry out a terrorist attack on such a scale in a country widely regarded as one of the world's most peaceful.
"Far too often, Muslims suffer unimaginable loss and pain in the places where they should feel safest", Trudeau said in a statement.
At a press conference Friday, Police Association President Chris Cahill said that for nearly all of the officers responding, "it will be the biggest and toughest thing they've had to deal with in their career", as NZ Herald reported.
Two armed guards escorted Tarrant into the court and he showed no expression as District Court Judge Paul Kellar read one charge of murder to him, the AP reported. A Turkish offcial said Ankara had also opened an inquiry after it emerged the man had made several visits to Turkey.
"This is a time of distress, we are in disarray, we don't know why this has happened", he said.
New Zealand Prime Minister Jacinda Ardern said the terror attack was "one of New Zealand's darkest days". "I'm advised that this was acquired in November of 2017", she said, saying the suspect also had a small arsenal of semi-automatic weapons.
A group called Syrian Solidarity New Zealand said Syrian refugees were inside the mosque when it came under attack. The U.S. stands by New Zealand for anything we can do. They included a two-year-old boy and a four-year-old girl, who is in critical condition.
There was a heavy police presence at the hospital where families of the more than 40 wounded had gathered. Funerals were planned on Saturday for some of the victims.
Jordan's King Abdullah II tweeted that "the heinous massacre against Muslims praying in peace in New Zealand is an appalling terrorist crime".
Another person who was earlier arrested was said to be a member of the public carrying a firearm who was trying to help.
"He had a big gun.He came and started shooting everyone in the mosque, everywhere", said the man, Ahmad Al-Mahmoud. "I was crammed at the door with others when a bullet hit my thigh and I fell on the floor".
Facebook said, that having been alerted by police, it had deleted the gunman's accounts "shortly after the livestream commenced".
Ardern said she had asked authorities to look into whether there was any activity on social media or elsewhere that could have alerted them ahead of the attack.
Christchurch is home to almost 400,000 people and is sometimes called the Garden City.
He was reprimanded without plea until his next appearance at New Zealand's High Court on April 5.
In his chilling The Great Replacement' document, the assassin laid bare his deranged ideology, claiming he was influenced by far-right Norwegian mass murderer Anders Breivik, who murdered 77 people in 2011.
Schools, institutions and workplaces went forced into lockdown for three and half hours on Friday afternoon as police hunted the shooters. It's a city in crisis mode yet again, just a few years on from the quakes that rattled Christchurch to its core.