A man who survived New Zealand's mosque attacks told a crowd of about 20,000 that he forgave the gunman who killed his wife and 49 other people.

Farid Ahmed was speaking at a national remembrance service held on Friday in Christchurch to commemorate those who died in the attacks two weeks ago.

It was the third major memorial held in the city since the attacks and a more formal occasion, with dozens of dignitaries from other countries attending, including Australian Prime Minister Scott Morrison.

The memorial featured musical guest Yusuf Islam, also known as Cat Stevens, who performed his song Peace Train.

Thousands stood in silence in Christchurch as the names of 50 people shot dead in two mosques were read, with speakers calling for the legacy of the tragedy to be a kinder, more tolerant New Zealand.

Prime Minister Jacinda Ardern, who wore a Maori cloak during the service, said the world had to end the vicious cycle of "extremism".

"Our challenge now is to make the very best of us a daily reality, because we are not immune to the viruses of hate, of fear. We never have been," said Ardern at service in Hagley Park, near the Al Noor mosque where more than 40 of the victims were killed by a white supremacist during Friday prayers on March 15.

"The answer to them lies in a simple concept that is not bound by domestic borders, that isn't based on ethnicity, power-base or even forms of governance. The answer lies in our humanity," she said.

Security was tight around the service and New Zealand remains on high-security alert. Police Commissioner Mike Bush said it was one of the largest security events ever conducted by the police in New Zealand. (Aljazeera)

• About 20,000 people attended the national remembrance service for victims of the mosque attacks [Jorge Silva/Reuters]