New Zealand prime minister Jacinda Ardern was greeted with a standing ovation as she took the stage to address a crowd of thousands gathered at Hagley Park for a nationwide remembrance service in honour of 50 people killed in the country’s worst terrorist attack.

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“The world has been stuck in a vicious cycle of extremism breeding extremism and it must end,” she said. “We cannot confront these issues alone, none of us can ... The answer lies in our humanity. But for now we will remember the tears of our nation and the new resolve we have formed.”

“We each hold the power – in our words, in our actions, in our daily acts of kindness – let that be the legacy of the 15th of March,” Ardern said.

She said to applause that New Zealanders had: “a responsibility to be the place that we wish to be. A place that is diverse, that is welcoming, that is kind and compassionate. Those values represent the very best of us. But even the ugliest of viruses can exist in places they are not welcome. Racism exists, but it is not welcome here.”

The service, known as Ko Tātou, Tātou We Are One, was broadcast live to many other events taking place around the country, to commemorate those who died two weeks ago when a gunman stormed two mosques in central Christchurch.

An estimated 20,000 people attended the event, including Australian prime minister Scott Morrison and other heads of state from the Pacific. Also in attendance were survivors of the attack and those who lost loved ones, as well as musician Yusuf Islam, formerly Cat Stevens, who performed at the event.

Facebook Twitter Pinterest Thousands of people gathered at Hagley Park near Al Noor mosque, where the shooting took place. Photograph: Jorge Silva/Reuters

Farid Ahmed, who survived the Al Noor mosque attack but whose wife Husna Ahmed died, said when he is asked why he has forgiven the attacker, he says Allah loves those who “control their anger and pardon their fellow human”.

His wife’s name was among the 50 read out by members of the Muslim community during a solemn part of the service.

Farid Ahmed said he wanted to offer his prayers for the people who were killed and also the people who survived. “I would like to honour you for your presence today ... I want to thank New Zealanders for coming together; for showing the world that New Zealand is a peaceful country.”

Christchurch mayor Lianne Dalziel told the crowd they all had a responsibility to question themselves about their own views and the “comfort we give to people who might harbour racist or hateful views”, adding: “We will not be defined by what happened on the 15th of March.”

President of the Muslim Council of Canterbury, Shaggaf Khan, said he was deeply saddened by the event but humbled by the outpouring of grief that followed.

Play Video 3:09 Yusuf Islam, survivors and leaders pay tribute at Christchurch memorial service – video

“That you did not leave us alone in our sadness. New Zealand responded in a way that none of us will ever forget – that let the world know who we really are.”

Before performing Peace Train, musician Yusuf Islam, said his heart went out to those affected and hoped the negative event would come to show the positives of the response.

“It’s only when good people stay sitting that evil rises ... we’ve seen the opposite in this country.”

The service was jointly led by the government, the City of Christchurch, local iwi Ngāi Tahu and the Muslim community.

Large swathes of the city were sealed off for the event, with police saying much planning had gone into the service and that public safety was the priority. The national threat level is still high.

At the remembrance event that was held at Auckland’s Eden Park stadium, All Black Sonny Bill Williams said he was a proud Muslim and New Zealander and commended the country for its reaction to the attack. “I just want to say to our New Zealand community, from my heart, the response has been beautiful.”

Williams, who has visited the injured in hospital, told the crowd. “Let’s keep leading the way for the world to see how it’s done.”