A man is to appear in court in New Zealand charged with murder after 49 people were killed in mass shootings at two mosques full of worshippers attending Friday prayers.
Authorities detained three other people - two men and a woman - and defused explosive devices in what appeared to be a carefully planned racist attack in Christchurch.
Prime Minister Jacinda Ardern called it "one of New Zealand's darkest days".
She said the incident represented "an extraordinary and unprecedented act of violence" and acknowledged that many of those affected may be migrants and refugees.
In addition to the dead, she said dozens of other people were seriously wounded.
"It is clear that this can now only be described as a terrorist attack," said Ms Ardern.
The Irish President said the attacks on two mosques in New Zealand have appalled people around the world.
In his statement, Michael D Higgins mourned the 49 people who lost their lives while attending Friday prayers in Christchurch.
"The attacks on mosques in New Zealand will have appalled people all over the world," he said.
"As President of Ireland, may I offer the sympathy of the people of Ireland to the families of the victims, and express the solidarity of the people of Ireland with the people of New Zealand at this time.
"This attack on innocent lives at spaces of worship for a religious community will be condemned by all those who believe in freedom and democratic values.
"I have conveyed deepest sympathies on behalf of the Irish people to Prime Minister (Jacinda) Ardern and to Governor-General (Patsy) Reddy."
Police took three men and a woman into custody after the shootings, which shocked people across the nation of five million people. One of the suspects was later charged with murder.
While there was no reason to believe there were more suspects, Ms Ardern said the national security threat level was being raised to the second-highest level.
Authorities have not specified who they detained, but said none had been on any watch list.
A man who claimed responsibility for the shootings left a 74-page anti-immigrant manifesto in which he explained who he was and the reasoning behind the attack. He said he was a 28-year-old white Australian and a racist.
Australian Prime Minister Scott Morrison confirmed that one of the four people detained was an Australian-born citizen.
Police Commissioner Mike Bush said on Friday night that a man had been charged with murder. He did not mention the other three suspects and did not say whether the same gunman was responsible for both attacks.
At a news conference Ms Ardern alluded to anti-immigrant sentiment as the possible motive, saying that, while many people affected by the shootings may be migrants or refugees, "they have chosen to make New Zealand their home, and it is their home. They are us."
On the suspects, she said: "These are people who I would describe as having extremist views that have absolutely no place in New Zealand."