A van exploded as police tried to defuse bombs found in it, near one of the churches attacked a day earlier.
Streets in the capital were mostly deserted, and many businesses remained shuttered as heavy police deployment continued in the country.
Authorities said the global intelligence agencies warned of attacks several times since the start of April, but Sri Lankan officials still failed to stop the deadly bombings, which killed almost 300 people.
Six blasts that claimed most of the victims took place Sunday morning when churches were crowded with Easter worshipers, and tourists and businessmen were sitting for breakfast in restaurants at luxury hotels. Gvernment is trying to ascertain how the suicide bombers were prepared. "We do not believe these attacks were carried out by a group of people who were confined to this country", Sri Lanka's Health Minister and Cabinet spokesman Rajitha Senaratne said on Monday.
Anura Ferdinand said that while all his family and friends are safe in the wake of the blasts, many other families now based in Canada are mourning the deaths of loved ones. "Persons named in intelligence reports are among those arrested".
Three churches that were targeted in that wave of blasts are Colombo's historic St Anthony's Shrine, the St Sebastian's church in the town of Negombo - north of the capital - and the Zion Church in the east-coast town of Batticaloa.
The emergency declaration, which comes into effect from midnight (19:30 GMT) on Monday, will give police and military extensive powers to detain and interrogate suspects without court orders.
"This is by all accounts an intelligence failure".
Prime Minister Ranil Wickremesinghe expressed anger that he was not made aware of the threat.
"We can confirm that several USA citizens were among those killed", Secretary of State Mike Pompeo said.
Sirisena and Wickremesinghe have been political rivals since the former attempted to replace him unsuccessfully four months ago, creating a divide in the government, according to political analysts. Late past year, Sirisena fired Wickremesinghe, setting off a constitutional crisisthat culminated in December, afterthe president was pressured to reinstate the prime minister. He had taken photographs of the US Consulate in Chennai, the Israeli Consulate in Bangalore and other vital installations, and studied their topography allegedly at the behest of Pakistani diplomat Amir Zubair Siddiqui, who was posted as visa counsellor in the Pakistani High Commission in Colombo.
Apart from hundreds of minority Catholics who were killed in the coordinated bombings at several churches, dozens of foreign nationals were also killed.
Most of those killed are thought to be Sri Lankan nationals but officials say at least 31 foreigners are among the dead including British, Indian, Danish, Saudi, Chinese and Turkish nationals.
"Still the investigations are going on", Mr Welianga said.
Sri Lanka's population is around 22 million, of which 70 per cent are Buddhists, 12.6 per cent are Hindus, 9.7 per of them are Muslims and 7.6 per cent Christians, according to the country's 2012 census. She says the family's in shock.