Senior cabinet minister Simon Birmingham says no Australians are among the more than 200 people killed, but one person could be wounded.

"We are not aware of any Australian loss of life. I understand there may be an Australian injured but I'm awaiting further details on that," he told ABC Radio National on Monday.

The Australian High Commission in Sri Lanka has been working to determine the whereabouts of Australians and monitor the situation.

Senator Birmingham said there was no indication about who was responsible for the atrocity, with no group yet to claim responsibility.

Sri Lankan Defense Minister Ruwan Wijewardena described the bombings as a terrorist attack by religious extremists, and police said 13 suspects were arrested, though there was no immediate claim of responsibility. Wijewardena said most of the blasts were believed to have been suicide attacks.

The government has acknowledged that it had "prior information" of attacks on churches involving a little known local Islamist group but didn't do enough about it. Attention is turning to an small Islamist group.

French agency Agence France Presse reported it had seen documents showing Sri Lanka's police chief Pujuth Jayasundara issued an intelligence alert to top officers 10 days ago, warning that suicide bombers planned to hit "prominent churches". He cited a foreign intelligence service as reporting that a little-known Islamist group was involved.

A Sri Lanka police spokesman said he was not aware of the intelligence report.

The explosions at three churches and three hotels - most of them in or around Colombo, the capital - collapsed ceilings and blew out windows, killing worshippers and hotel guests in one scene after another of smoke, soot, blood, broken glass, screams and wailing alarms. Victims were carried out of blood- spattered pews.

"People were being dragged out," said Bhanuka Harischandra, of Colombo, a 24-year-old founder of a tech marketing company who was going to the city's Shangri-La Hotel for a meeting when it was bombed. "People didn't know what was going on. It was panic mode."

He added: "There was blood everywhere."

Most of those killed were Sri Lankans. But the three hotels and one of the churches, St. Anthony's Shrine, are frequented by foreign tourists. Government officials said 32 foreigners were killed and 30 injured in the explosions that tore through congregations and gatherings in hotels in Colombo, Negombo and Batticaloa.

They included five British people, two of whom had dual US citizenship, and three Indians, according to officials in those countries.

Also among the fatalities were three people from Denmark, two from Turkey, and one from Portugal, Sri Lankan officials said. There were also Chinese and Dutch among the dead, according to media reports.

The Sri Lankan government imposed a nationwide curfew from 6pm to 6am and blocked Facebook and other social media, saying it needed to curtail the spread of false information and ease tension in the country of about 21 million people.

Prime Minister Ranil Wickremesinghe said he feared the massacre could trigger instability in Sri Lanka, and he vowed to "vest all necessary powers with the defense forces" to take action against those responsible.

The Archbishop of Colombo, Cardinal Malcolm Ranjith, called on Sri Lanka's government to "mercilessly" punish those responsible "because only animals can behave like that."

In a sign that the attacks could lead to communal violence, police reported on Sunday night there had been a petrol bomb attack on a mosque in the northwestern district of Puttalum and arson attacks on two shops owned by Muslims in the western district of Kalutara.

Police spokesman Ruwan Gunasekara said at least 207 people were killed and 450 wounded. He said police found a safe house and a van used by the attackers.

The first six explosions were all reported within a short period in the morning just as church services were starting.

The explosion at the Tropical Inn happened later and there was an eighth explosion at the house that was the subject of the police raid in Colombo.

The scale of the bloodshed recalled the worst days of Sri Lanka's 26-year civil war, in which the Tamil Tigers, a rebel group from the ethnic Tamil minority, sought independence from the Buddhist-majority country. The Tamils are Hindu, Muslim and Christian.

Sri Lanka, situated off the southern tip of India, is about 70 percent Buddhist, with the rest of the population Muslim, Hindu or Christian. While there have been scattered incidents of anti-Christian harassment in recent years, there has been nothing on the scale of what happened Sunday.

There is also no history of violent Muslim militants in Sri Lanka. However, tensions have been running high more recently between hard-line Buddhist monks and Muslims.

Two Muslim groups in Sri Lanka condemned the church attacks, as did countries around the world, and Pope Francis expressed condolences at the end of his traditional Easter Sunday blessing in Rome.

"I want to express my loving closeness to the Christian community, targeted while they were gathered in prayer, and all the victims of such cruel violence," Francis said.

Six nearly simultaneous blasts took place in the morning in Colombo at St. Anthony's Shrine - a Catholic church - and the Cinnamon Grand, Shangri-La and Kingsbury hotels. After a lull of a few hours, two more explosions occurred at St. Sebastian Catholic church in Negombo, a mostly Catholic town 23 miles (37 kilometers) north of Colombo, and at the Protestant Zion church in the town of Batticaloa, about 190 miles (300 kilometers) to the east.

Three police officers were killed while conducting a search at a suspected safe house in Dematagoda, on the outskirts of Colombo, when its occupants apparently detonated explosives to prevent arrest, Wijewardena said.

Local TV showed the Shangri-La's second-floor restaurant was gutted, with the ceiling and windows blown out. Loose wires hung down and tables were overturned in the blackened space. From outside the police cordon, three bodies could be seen covered in white sheets.

Foreign tourists hurriedly took to their cellphones to text family and loved ones that they were OK. Visitors from around the world come to Sri Lanka to see elephants, tea plantations, ancient Buddhist monuments and other sights.

"I had a sense that the country was turning the corner, and in particular those in the tourism industry were hopeful for the future," said tourist Peter Kelson, a technology manager from Sydney. "Apart from the tragedy of the immediate victims of the bombings, I worry that these terrible events will set the country back significantly."

Locals who work in Sri Lanka's vital tourism industry were shocked and upset by the bloodshed.

"After so many years, we've started again," said Gamini Francis, 56, a long-time hotel worker. "A lot of people are going to lose their jobs. 100 per cent sure. It's tragic. Crazy people killing innocent people."

Sri Lankan forces defeated the Tamil Tiger rebels in 2009, ending a civil war that took over 100,000 lives, with both sides accused of grave human rights violations.

Harischandra, who witnessed the attack at the Shangri-La Hotel, said there was "a lot of tension" after the bombings, but added: "We've been through these kinds of situations before."

He said Sri Lankans were "an amazing bunch" and noted that his social media feed was flooded with photos of people standing in long lines to give blood.

AP