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".
"We must stand together to make sure that no one should ever have to practise their faith in fear".
House Speaker Nancy Pelosi, D-Calif., called the bombings "heartbreaking" for a country that suffered from years of bloody civil war.
The attacks came as a shock to the country, which thought it had put decades of civil war behind it.
The almost simultaneous first six blasts Sunday morning toppled ceilings and blew out windows at a famous Catholic church in Colombo, the capital, and at three luxury hotels in the city.
"When crime division officials started questioning the people in the house, two explosions occurred", police spokesman Ruwan Gunasekara said.
Earlier in the day, following reports that seven individuals had been arrested, Defense Minister Ruwan Wijewardene called on the media to not identify the attackers involved by name.
At the Shangri-La, witnesses said they heard two loud blasts and that staff reported some people had been killed.
The explosions blew out the tiled roofs of churches and hotel windows, killing worshippers and hotel guests in the process.
Tillekeratne said the ground was covered in rubble and shattered glass with about 30 bodies lying in the church area.
"It's striking that in nearly three decades of war between the Tamils and government forces, foreign tourists were never targeted".
Sri Lanka's foreign secretary, Ravinath Aryasinghe, said the bodies of 27 foreigners were recovered from the blast sites.
In a statement to the Guardian, a Facebook spokesperson said they were "aware of the government's statement regarding the temporary blocking of social media platforms".
Prime Minister Scott Morrison said in a statement: "To the handsome people of Sri Lanka, Australia sends its heartfelt sympathies and our prayers and our support - and our offer to do whatever we can to support you in this awful time of need".
A Sri Lanka state-run newspaper, the Daily News, reported that more than 500 people have been hospitalised with injuries caused by the blasts. During the war, the Tigers and other rebels carried out a multitude of bombings.
Sri Lanka is a predominantly Buddhist nation but is also home to significant Hindu, Muslim and Christian communities. While there have been scattered incidents of anti-Christian harassment in recent years, there has been nothing on the scale of what happened Sunday.
Tourists were also targeted in the attacks, Sri Lanka High Commissioner Gunasekera told CNN.
"Terming them as cold-blooded and pre-planned barbaric acts, he pointed out that these attacks were another grim reminder of the most serious challenge posed to the entire humanity by terrorism in our region and the entire world", the statement added.
Sri Lanka's minority Christian community appeared to be the main target of Sunday's attack.