Easter Sunday attacks which left more than 100 dead in Sri Lanka were "truly appalling", Theresa May has said.
The Prime Minister said "no one should ever have to practise their faith in fear" following blasts which tore through three churches and three hotels.
The sites were hit with near-simultaneous explosions, with two suspected to be caused by suicide bombers.
Reports from Colombo say 138 people have died while more than 500 are in hospital.
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."
The UK's High Commissioner to Sri Lanka James Dauris was in Colombo with his family at a church service which was cut short by the attacks.
He tweeted: "Our prayers for the victims of these evil attacks, and for their families. Our thoughts are with the medical staff, police and all involved in the response."
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."
A spokesman for the Foreign and Commonwealth Office said: "We are aware of reports of a number of explosions in Sri Lanka, including Colombo, and we are urgently seeking information from the local authorities.
"British nationals in Sri Lanka should follow the instructions of the local authorities and check FCO travel advice for updates."