Prime Minister Jacinda Ardern spoke at the televised event, along with Muslim leaders and a survivor of the attack.
Cat Stevens, the British singer who converted to Islam in the 1970s, also performed.
More than 20,000 people attended the event at the city’s Hagley Park, amid tight security.
Addressing the crowd in the park near the Al Noor mosque, where more than 40 people were killed earlier this month, Ms Ardern said New Zealand had “a responsibility to be the place we wish to be”.
“We are not immune to the viruses of hate, of fear, of other. We never have been,” she said, “but we can be the nation that discovers the cure”.
In all, 50 people died on 15 March when a white supremacist attacked two mosques in Christchurch, while livestreaming part of his attack on Facebook.
Ms Arden described how the world had been “stuck in a vicious cycle of extremism breeding extremism” but said “the answer lies in our humanity”.
Wearing a Maori cloak, she was joined by dozens of government representatives from around the world.
Farid Ahmed, who survived the attack by a far-right gunman but whose wife Husna was killed, made a plea for peace. He said he had forgiven the gunman.
“I don’t want a heart that is boiling like a volcano,” he said. “I want a heart that will be full of love and care, and will have mercy.”
Cat Stevens, whose Muslim name is Yusuf Islam, sang his songs Peace Train and Don’t be Shy. “It’s only when good people stay sitting that evil rises,” he said. “We’ve seen the opposite in this country.”
Shaggaf Khan, president of the Muslim Council of Canterbury, praised New Zealand’s response to the attacks and said he felt “ultimately hopeful”.
“From this hate, how much love has been shared? Out of this darkness, how much light has been spread?” he asked the crowd.
The mayor of Christchurch, Lianne Dalziel, called on social media platforms to “take more responsibility”. The shootings were live-streamed on Facebook and shared worldwide before the social media network could remove the footage.
“We each have a responsibility to ask the hard questions of ourselves about what comfort we give to people who might harbour racist or hateful views,” she said.
All 50 names of the victims who died in the attacks were read out at the event by members of the city’s Muslim community. The victims included men, women and children from across the world. The youngest victim was just three years old.
Many had moved to New Zealand in the hope of a better life and some were refugees. Ms Ardern described the attacks as one of the nation’s “darkest days”.
Australian Brenton Tarrant, 28, a self-proclaimed white supremacist, has appeared in court on a single murder charge and is expected expected to face further charges.
Since the mass shooting, Ms Ardern has announced a ban on all types of semi-automatic weapons and assault rifles.
She announced on Thursday that Prince William will visit New Zealand next month and meet survivors of the attack.