Forty nine people were killed and more than 20 seriously wounded in the attacks in Christchurch, New Zealand Police Commissioner Mike Bush said.
Video footage widely circulated on social media, apparently taken by a gunman and posted online live as the attack unfolded, showed him driving to one mosque, entering it and shooting randomly at people inside.
"It is clear that this can now only be described as a terrorist attack," Prime Minister Jacinda Ardern said, adding it marked one of New Zealand's darkest days.
"Many of those who would have been affected by this shooting may be migrants to New Zealand. They may even be refugees here. They have chosen to make New Zealand their home and it is their home," she said.