New Zealand, March 15 -- Mass shootings at two mosques full of worshippers attending Friday prayers killed 40 people on what the prime minister called "one of New Zealand's darkest days," as authorities detained four people and defused explosive devices in what appeared to be a carefully planned attack.

Prime Minister Jacinda Ardern said the events in Christchurch represented "an extraordinary and unprecedented act of violence" and acknowledged many of those affected may be migrants and refugees. In addition to the dead, she said more than 20 people were seriously wounded.

"It is clear that this can now only be described as a terrorist attack," Ardern said.

Police took three men and a woman into custody after the shootings, which shock...