Five Britons are among more than 200 people killed in a series of bombings which ripped through churches and luxury hotels in Sri Lanka.
Prime Minister Theresa May said the Easter Sunday massacre was "truly appalling", and "no one should ever have to practise their faith in fear".
Sri Lanka's foreign ministry said three Britons and two holding joint US and British nationalities were killed. A Blackpool businessman was caught up in the bombings but escaped unhurt, it was understood.
James Dauris, UK's High Commissioner in Sri Lanka, visited injured Britons in hospital and condemned the "senseless attack".
Six nearly simultaneous explosions at churches and hotels killed scores of people in Colombo, Negombo and Batticaloa.
Hours later, there were further explosions in Dehiwala and Dematagoda on the outskirts of Colombo.
The authorities said 207 were killed and 450 injured in the attacks, most of which were being blamed on suspected suicide bombers.
No one has taken responsibility for the killings, but officials say seven suspects have been arrested.
The Easter attacks are the worst bloodshed Sri Lanka has seen since its brutal civil war ended a decade ago.
Mr Dauris said: "I've been speaking this afternoon with Brits in hospital who have been affected by today's senseless attacks.
"My team's and my thoughts go out to all those people who are suffering as a result of the deplorable violence Sri Lanka has witnessed this Easter Sunday."
Mrs May said: "The acts of violence against churches and hotels in Sri Lanka are truly appalling, and my deepest sympathies go out to all of those affected at this tragic time.
"We must stand together to make sure that no one should ever have to practise their faith in fear."
Britons in Sri Lanka who need help were urged to call the High Commission in Colombo on +94 11 5390639, while people in the UK worried about friends or family should call the Foreign Office on 020 7008 1500.
Labour leader Jeremy Corbyn called for "unity, love and respect" to combat hatred.
He said: "I'm appalled by the horrific attacks in Sri Lanka, on Easter Sunday, the most important day in the Christian calendar.
"I stand with the victims, their families, the people of Sri Lanka and Christians around the world. We must defeat this hatred with unity, love and respect."
Foreign Secretary Jeremy Hunt said the attacks were "horrifying", and "to target those gathered for worship on Easter Sunday is particularly wicked".
Archbishop of Canterbury Justin Welby said: "On this holy day, let us stand with the people of Sri Lanka in prayer, condolence and solidarity as we reject all violence, all hatred and all division."
Sri Lanka's prime minister Ranil Wickremesinghe condemned "the cowardly attacks on our people".
In Colombo, St Anthony's Shrine and the Cinnamon Grand, Shangri-La and Kingsbury hotels were targeted in the first wave of explosions.
Other blasts were reported at St Sebastian's Church in Negombo, a majority Catholic town north of Colombo, and at Zion Church in the eastern town of Batticaloa.
Julian Emmanuel and his family, from Surrey, were staying at the Cinnamon Grand when the bomb went off.
He told the BBC: "We were in our room and heard a large explosion. It woke us up. There were ambulances, fire crews, police sirens.
"I came out of the room to see what's happening, we were ushered downstairs.
"We were told there had been a bomb. Staff said some people were killed. One member of staff told me it was a suicide bomber."