Sadiq Khan, the mayor of London, has tweeted his condolences to the victims of the attack, and confirmed that there will be "highly visible" and armed police around mosques in the UK capital on Friday.
"I want to reassure the Muslim communities in London. I have been in touch with the Met Police. There will be highly visible policing around mosques today, as well as armed response officers, as Londoners go to pray," he said.
London has a large Muslim community. In 2017, one person was killed and several were injured when man drove a van into pedestrians near Finsbury Park Mosque in north London.
Australian Prime Minister Scott Morrison says he has asked for flags to be flown at half-mast out of respect for those killed in the attack.
"Australians stand with all New Zealanders today during this dark time where hate and violence has stolen their peace and innocence. Kia kaha (stay strong)," Morrison tweeted earlier.
"New Zealand, like Australia, is home to people from all faiths, cultures and backgrounds. There is absolutely no place in either of our countries for the hatred and intolerance that has bred this extremist, terrorist violence and we condemn it," he added.
Out of respect and in condolence for all those killed in the terrorist attack in New Zealand, I have asked for flags to be flown at half-mast. pic.twitter.com/0qgIrmdgoH
— Scott Morrison (@ScottMorrisonMP) March 15, 2019
Muslim-majority nations Turkey, Malaysia and Pakistan have condemned the twin attacks on mosques in the New Zealand city of Christchurch on Friday.
Turkish President Recep Tayyip Erdogan said the terrorist attacks were examples of "rising racism and Islamophobia."
"On behalf of my country, I offer my condolences to the Islamic world and the people of New Zealand, who have been targeted by this deplorable act," he said in a statement.
Pakistan Prime Minister Imran Khan said he was "shocked" by the attack. "This reaffirms what we have always maintained: That terrorism does not have a religion. Prayers go to the victims and their families," he said on his official Twitter.
In a separate statement, the Malaysian government said two of their citizens had been injured in the attack.
"Malaysia condemns in the strongest terms this senseless act of terror on innocent civilians and hopes that those responsible for this barbaric crime be brought to justice," a statement from the foreign ministry said.