Explosions at three Sri Lankan churches and three luxury hotels occurred about 2.30pm AEST. A seventh explosion, at a fourth hotel, was reported about 7pm. Police then reported an eighth blast.

Sri Lanka's defence minister said seven people had been arrested in relation to the blasts. Two of the blasts were suspected to have been carried out by suicide bombers, according to the official, who spoke on condition of anonymity because he was not authorised to speak with reporters.

Worshippers and hotel guests were among the dead, the official said.

St. Anthony's Shrine and the three hotels where Sunday's blasts took place are in Colombo, the capital, and are frequented by foreign tourists.

The three hotels hit were the Shangri-La Colombo, Kingsbury Hotel and Cinnamon Grand Colombo.

The Shangri-La's second-floor restaurant was gutted in the blast, with the ceiling and windows blown out. Loose wires hung and tables were overturned in the blackened space.

A police magistrate was at the hotel to inspect the bodies recovered from the restaurant. From outside the police cordon, three bodies could be seen covered in white sheets.

Alex Agieleson, who was near the shrine, said buildings shook with the blast, and that a number of injured people were carried away in ambulances.

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.

St. Sebastian's church posted pictures of destruction inside the church on its Facebook page, showing blood on pews and the floor, and requested help from the public.

The explosion ripped off the roof and knocked out doors and windows at St. Sebastian's, where people carried the wounded away from blood-stained pews, TV footage showed.

Nine foreigners were among the dead, officials said. Chinese media is reporting one of the nine foreigners to have died is a Chinese national.

There were no immediate claims of responsibility for the attacks in a country which was at war for decades with Tamil separatists until 2009 during which bomb blasts in the capital were common.

Prime Minister Ranil Wickremesinghe has convened Sri Lanka's top military officials at an emergency meeting of the National Security Council.

"I strongly condemn the cowardly attacks on our people today. I call upon all Sri Lankans during this tragic time to remain united and strong," he said in a tweet.

"Please avoid propagating unverified reports and speculation. The government is taking immediate steps to contain this situation."

President Maithripala Sirisena said he had ordered the police special task force and military to investigate who was behind the attacks and their agenda.

The military had been deployed, according to a military spokesman, and security stepped up at Colombo's international airport.

The Sri Lankan government declared a curfew and shut down access to major social media and messaging services to prevent misinformation and rumours.

Australian Prime Minister Scott Morrison described the bombings as "absolutely devastating".

"As Easter Sunday draws to a conclusion here in Australia, our heart goes out to those Christians and all of those other innocents who have been slaughtered today in this horrific terrorist attack," Mr Morrison said.

Sri Lanka has long faced a bitter ethnic divide between the majority Sinhalese and the minority Tamils, fueling the civil war as Tamil militants tried to carve out their own homeland.

But in the years since the war ended in 2009, a religious divide has grown, with the rise of Buddhist nationalist groups that stoke anger against the minority Muslims, saying they are stealing from Buddhist temples or desecrating them, or forcing people to convert to Islam. Muslims also own many of Sri Lanka's small shops, and many Muslims suspect small-town jealousy has led to some attacks.

Sinhalese are overwhelmingly Buddhists, while Tamils are mostly Hindu, Muslim and Christian.

Last year, there were 86 verified incidents of discrimination, threats and violence against Christians, according to the National Christian Evangelical Alliance of Sri Lanka (NCEASL), which represents more than 200 churches and other Christian organisations.

This year, the NCEASL recorded 26 such incidents.

Out of Sri Lanka's total population of about 22 million, 70 per cent are Buddhist, 12.6 per cent Hindu, 9.7 per cent Muslim, and 7.6 per cent Christian, according to the country's 2012 census.

AP with Reuters