Sri Lanka fell silent for three minutes Tuesday on a day of national mourning to honour more than 300 people killed in suicide bomb blasts that have been blamed on a local Islamist group.

Flags were lowered to half mast on government buildings, and people bowed their heads and reflected silently on the violence that has caused international outrage.

The silence began at 8:30 am (0300 GMT), the time that the first of six bombs detonated on Sunday morning, unleashing carnage at high-end hotels and churches packed with Easter worshippers.

Shortly after the silence was observed, a police spokesman said the death toll had risen to 310, with several people dying of their injuries overnight.

The first memorial services for the victims, among them dozens of foreigners, were being held Tuesday, hours after the government imposed a state of emergency and said an Islamist group was behind the violence.

At St Anthony's Shrine in Colombo -- where scores died as they gathered for Easter Sunday prayers -- a few dozen people held candles and prayed silently, palms pressed together.

And at St Sebastian's Church in Negombo, north of the capital, an elderly man wept uncontrollably by the coffin bearing the body of his wife.

Police said that 40 people were now under arrest over the suicide bomb attacks -- the worst atrocity since Sri Lanka's civil war ended a decade ago.

The attacks were also the worst ever against the country's small Christian minority, who make up just seven percent of the 21 million population.

Investigators are now hunting for clues on whether the local Islamist group named as the chief suspect -- National Thowheeth Jama'ath (NTJ) -- received "international support", said cabinet minister and government spokesman Rajitha Senaratne.

He said it was not possible for such "a small organisation" to carry out such well co-ordinated suicide strikes.

President Maithripala Sirisena's office said there was intelligence that "international terror groups" were behind the local perpetrators and that he would seek foreign help to investigate.

The state of emergency, which gave police and the military special powers to counter militant strikes, came into force at midnight (1830 GMT Monday). Suspects can be detained without a court order.

A second straight night-time curfew was also put in place, but lifted before dawn.

Parliament will hold its first session since the attacks on Tuesday, amid questions on whether the blasts could have been prevented.

Officials are investigating why more precautions were not taken after an April 11 warning from Sri Lanka's police that a "foreign intelligence agency" had reported the NTJ planned suicide attacks on churches.

Spokesman Senaratne said the warning was not passed on to Prime Minister Ranil Wickremesinghe or other top ministers.

- Tensions high -

Tensions remained high and security heavy after a bomb discovered by police on Monday near one of the targeted churches blew up before police could defuse it. Although there was a powerful blast, no injuries were reported.

Police also found 87 bomb detonators at a Colombo bus station.

More details have begun to emerge about some of the foreigners killed in the blasts.

The United States reported at least four Americans killed -- including a child -- and the Netherlands raised their toll to three.

A Danish billionaire lost three of his children in the attacks, a spokesman for his company said.

Eight Britons, eight Indians and nationals from Turkey, Australia, France, Japan and Portugal, were also killed, according to Sri Lankan officials and foreign governments.

The suicide bombers hit three Colombo luxury hotels popular with foreign tourists -- the Cinnamon Grand, the Shangri-La and the Kingsbury -- and three churches: two in the Colombo region and one in the eastern city of Batticaloa.

Two additional blasts were triggered as security forces carried out raids searching for suspects.

Interpol said it was deploying investigators and specialists to Sri Lanka, and the US State Department warned of possible further attacks in a travel advisory.

- Memories of civil war -

Ethnic and religious violence has plagued Sri Lanka for decades, with a 37-year conflict with Tamil rebels followed by an upswing in recent years of clashes between the Buddhist majority and Muslims.

Burials were expected to begin for some of the dead on Tuesday.

At St Sebastian's, the atmosphere was heavy with grief as coffins were brought in the grounds one at a time for services.

"There are so many bodies that we can't accommodate them all at once," Anthony Jayakody, auxiliary bishop of Colombo, told AFP.

The attacks have sparked local and international outrage, and have been condemned by Sri Lankan Muslim groups, with one urging the "maximum punishment for everyone involved in these dastardly acts".