USA president Donald Trump said that his "warmest sympathy and best wishes goes out to the people of New Zealand after the disgusting massacre in the mosques,".
Following the incident, The Stuff news portal reported that a Christchurch hospital was expecting to receive from 40 to 50 people injured in the shootings.
Ms 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.
"With this attack, hostility towards Islam that the world has been has been idly watching and even encouraging for some time, has gone beyond the boundaries of individual harassment to reach the level of mass killing", said Turkish President Recep Tayyip Erdogan.
In the attack - which took place at two mosques in Christchurch, New Zealand, at least 49 people were killed.
Australian Prime Minister Scott Morrison confirmed the shooter was Australian and described him as an "extremist, right-wing, violent terrorist".
Police have said a man in his late 20s has been charged with murder and will appear at the Christchurch court on Saturday morning.
Bush later stated that a number of improvised explosive devices, which had been attached to vehicles, had been defused after the shootings.
A forth person arrested on Friday who was in possession of a firearm, but with the intention of assisting police, has since been released.
French President Emmanuel Macron echoed Merkel's message, condemning an "odious attack" and saying France "stands against any form of extremism".
The Queen was joined in her condolences by British Prime Minister Theresa May.
"I blame these increasing terror attacks on the current Islamophobia post-9/11 (where) 1.3 billion Muslims have collectively been blamed for any act of terror", Pakistani Prime Minister Imran Khan posted on social media.
The secretary-general of the 57-nation Organization of Islamic Cooperation, Youssef al-Othaimeen, said in a statement that the attack "served as a further warning on the obvious dangers of hate, intolerance, and Islamophobia".
My first thoughts go to the relatives and families of the victims whose emotion and sadness we share.
Additional police will also be deployed to religious building and services over the weekend, Police Commissioner Ian Stewart said, according to the News.com.au website.
The Muslim community does, however, face challenges in New Zealand.
"Police can now confirm the lockdown of schools throughout Christchurch has been lifted", the police said in a statement.
"Feeling very sick, that person is brainless and a savage", said one Indonesian Twitter user who identified himself as Farhan Adhitama.
"I want to reassure the Muslim communities in London", said Khan.
"The US stands by New Zealand for anything we can do. Our heartfelt Duas go out to all of the victims, their families and the Muslim communities that have been tragically affected by the gunmen who killed our brothers and sisters half way across the world", the center stated in a post on Facebook.
A second man said he quickly realised the seriousness of the incident when he saw people covered in blood.
The Muslim community in Chicago is taking measures to ensure safety here, after 49 people were killed in coordinated terrorist attacks at two mosques in New Zealand.