In a tweet she sent "deepest condolences to the people of New Zealand after the horrifying terrorist attack in Christchurch".
She added: "My thoughts are with all of those affected by this sickening act of violence."
Foreign Secretary Jeremy Hunt said: "Our hearts go out to the people of New Zealand following the news of this terrible act in Christchurch.
" NZ is one of the most peaceful, peace-loving and generous nations in the world.
"Your friends in the UK stand with you today in deepest sympathy."
Home Secretary Sajid Javid tweeted he was "absolutely heartbroken to hear about this attack on peaceful worshippers".
Mr Javid added: "We stand with New Zealand and Muslims across the world against all forms of racism and anti-Muslim hatred. We will not let extremists divide us #ChristchurchAttack.
"A horrific terror attack. We will never let the terrorists win and divide our communities. My thoughts and prayers with the victims and families of all those affected."
Labour leader Jeremy Corbyn said on Twitter: "My heart goes out to the victims of the horrific terror attack in New Zealand. We stand in solidarity with the Muslim community in Christchurch and around the world.
"We must defeat the bigotry which fuels such hatred and violence."
Baroness Warsi, a former Minister of State for Faith and Communities, said violence started with the "demonisation and vilification of communities".
She tweeted: "I urge a moment of self reflection by politicians on how THEY impact public discourse around Muslims."
UK-based Anti-Islamaphobia group Tell MAMA said the attack showed how "Anti-Muslim hatred is fast becoming a global issue and a binding factor for extremist far right groups".
Iman Atta from the group said: "We are appalled to hear about the mass casualties in New Zealand.
"The killer appears to have put out a 'manifesto' based on white supremacist rhetoric which includes references to anti-Islamic comments.
"He mentions 'mass immigration' and 'an assault on our civilisation' and makes repeated references to his 'white identity'.
"The killer also seems to have filmed the murders adding a further cold ruthlessness to his actions.
"We have said time and time again that far right extremism is a growing problem and we have been citing this for over six years now.
"That rhetoric is wrapped within anti-migrant and anti-Muslim sentiment.
"Anti-Muslim hatred is fast becoming a global issue and a binding factor for extremist far right groups and individuals.
"It is a threat that needs to be taken seriously"
Mohammed Shafiq of the Ramadhan Foundation said the attacks were by far right extremists "inspired by their hatred of Muslims and Islam".
He said: "An attack on a place of worship is an attack against all faiths, I'm confident that in the days ahead you will see all communities come together.
"This sadly is not an isolated incident, for many years commentators, the media, politicians and far right extremists have dehumanised Islam and Muslims, have perpetuated the narrative that Muslims are responsible for the 'ills of the world' and that our lives are not worthy of defence."
He added: "We call on Governments around the world to step up security for Mosques and Islamic Centres, the days ahead will be full of mourning and sadness but together as one human family we will prevail."