WELLINGTON, New Zealand — A gunman opened fire on two mosques in central Christchurch, New Zealand, on Friday, killing multiple people in an afternoon slaughter that in part was broadcast live online after the publication of a white supremacist manifesto.

The police said a “significant” number of people were killed, shaking a country with little history of mass shooting in what the prime minister called “an extraordinary and unprecedented act of violence.”

Some of the shootings in the city of Christchurch were streamed on Facebook, a grim development in terrorism that raised questions about the ability of tech companies to block violent content.

The police said three men and one woman were in custody, but they were unsure if there were others involved. The country’s police commissioner, Mike Bush, said that a number of explosive devices were found in vehicles stopped by police.