Britons are thought to be among those caught up in a series of explosions which ripped through churches and hotels in Sri Lanka, killing more than 100 people.
Theresa May said the Easter Sunday massacre was "truly appalling" and "no one should ever have to practise their faith in fear".
The UK's High Commissioner to Sri Lanka, James Dauris, said: "We understand that some British citizens were caught in the blasts but we are unable to say how many people are, or might have been, affected."
He urged Britons to get in touch with members of their family to let them know they were safe.
Six nearly simultaneous explosions at churches and hotels killed at least 138 people in Colombo, Negombo and Batticaloa, with hundreds more injured.
Hours later, two more explosions occurred - a blast at a guesthouse in Dehiwala killed two and an eighth incident occurred in Dematagoda on the outskirts of Colombo.
At least nine foreign nationals are thought to be among the dead.
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."
Mr Dauris was in Colombo with his family at a church service which was cut short by the attacks.
In a video message he said: "Earlier today a series of explosions took place in hotels and churches in Colombo, in Negombo and in Batticaloa on Sri Lanka's east coast.
"The churches were busy with Easter Sunday worshippers, the hotels with local people and foreign visitors.
"Reports suggest that several dozen people have been killed and hundreds injured in these evil and senseless attacks."
Mr Dauris and his consular team were visiting one of the main hospitals in Colombo where casualties had been taken.
He said: "If you are affected, or are concerned about someone who you think might be affected, please help us to help you.
"If you are in Sri Lanka please follow the instructions given to you by local authorities and hotel security staff.
"Please follow our travel advice and keep checking for updates."
Those 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 he was "deeply shocked and saddened" by the "horrifying attacks".
He added: "To target those gathered for worship on Easter Sunday is particularly wicked."
In Colombo, St Anthony's Shrine and the Cinnamon Grand, Shangri-La and Kingsbury hotels were targeted.
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."