Muslim women have been ordered to stop wearing veils in public and Sri Lanka's Catholic churches have been closed as the country imposes strict security measures in the aftermath of the Easter suicide bombings.

Key points: Sri Lankan President Maithripala Sirisena banned all kinds of face coverings that conceal people's identities

Sri Lankan President Maithripala Sirisena banned all kinds of face coverings that conceal people's identities The emergency law prevents Muslim women from veiling their faces

The emergency law prevents Muslim women from veiling their faces The country's Catholic churches were closed after warnings more militants remained on the loose

The closing of all of Sri Lanka's Catholic churches — an extraordinary measure unheard of in the church's centuries on this island off the southern tip of India — came after local officials and the US Embassy in Colombo warned more militants remained on the loose with explosives.

Islamic State has claimed responsibility for the Easter suicide bombings, which were aimed at churches and hotels and killed more than 250 people.

President Maithripala Sirisena also banned all kinds of face coverings that may conceal people's identities.

The emergency law, which takes effect from Monday (local time), prevents Muslim women from veiling their faces.

The decision came after the cabinet had proposed laws on face veils at a recent meeting.

Sri Lankan Catholics pray standing on a road during a brief holy mass held outside the exploded St Anthony's Church. ( AP: Eranga Jayawardena )

It had deferred the matter until talks with Islamic clerics could be held, on the advice of Prime Minister Ranil Wickremesinghe.

With churches closed, many across the nation knelt before their televisions as Cardinal Malcolm Ranjith, the Archbishop of Colombo, delivered a homily before members of the clergy and the country's leaders in a small chapel at his residence in the capital.

In a rare show of unity, Mr Sirisena, Mr Wickremesinghe and Opposition Leader Mahinda Rajapaksa had attended the Mass in person.

Their political rivalry and government dysfunction are blamed for a failure to act upon near-specific information received from foreign intelligence agencies that preceded the bombings, which targeted three churches and three luxury hotels.

Police said they had arrested 48 suspects over the past 24 hours as checkpoints mounted by all of Sri Lanka's security forces sprung up across the country of 21 million people.

Those arrested included two men authorities recently appealed to the public to locate.

The Government also warned it would crack down on those spreading false information and making inflammatory remarks.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 1 minute 9 seconds 1 m 9 s Sri Lanka's capital is rocked by explosions set off during Easter Sunday services

Before services began, the Islamic State group claimed three militants who blew themselves up on Friday night after exchanging fire with police in the country's east.

Investigators sifting through that site and others uncovered a bomb-making operation capable of spreading far more destruction.

"This is a time our hearts are tested by the great destruction that took place last Sunday," Cardinal Ranjith told those watching across the nation.

"This is a time questions such as, 'Does God truly love us?' 'Does he have compassion toward us?' Can arise in human hearts."

National Tawheed Jamaath declared a terror group

A day after authorities declared National Tawheed Jamaath and another organisation terror groups over the bombings, police entered the organisation's Kattankudy mosque and stopped an interview among foreign journalists and mosque officials.

Later, a senior police officer dispersed journalists waiting outside, saying authorities were conducting a "cordon and search operation".

Police then left, locking up the mosque just before afternoon prayers were to start.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 37 seconds 37 s Sri Lankan police seized bomb-making materials in a raid. ( ABC News )

Authorities banned National Tawheed Jamaath over its ties to Mohammed Zahran, the alleged mastermind of the Easter Sunday bombings.

Zahran and masked others had pledged their loyalty to IS leader Abu Bakr al-Baghdadi before carrying out the attacks, showing the danger the extremist group poses even after losing all its territory in Iraq and Syria.

On Friday night, a confrontation with police sparked a firefight with the militants in Kalmunai, 225 kilometres north-east of Colombo.

Sri Lanka's military said the gunfire and later suicide blasts killed 15 people, including six children.

In a statement carried by IS's Aamaq news agency, it identified the bombers by their noms du guerre as Abu Hammad, Abu Sufyan and Abu al-Qa'qa.

It said they opened fire with automatic weapons and, "After exhausting their ammunition, detonated … their explosive belts".

Police spokesman Ruwan Gunasekara said a woman and a four-year-old child found wounded after the gun battle have been identified as Zahran's wife and daughter.

AP