Sri Lanka has banned about 200 Muslim clerics as a security operation and investigation continue into the Easter Sunday massacre that killed more than 250 people.

The government will deport more than 600 foreign nationals, claiming that their visas have expired and they have overstayed on the Indian Ocean island. The clerics are part of this group.

Home Affairs Minister Vajira Abeywardena said the clerics had entered the country legally, but amid a security crackdown and the discovery of their expired visas, fines were imposed and they were expelled.

"Considering the current situation in the country, we have reviewed the visa system and took a decision to tighten visa restrictions for religious teachers," Mr Abeywardena said.

"Out of those who were sent out, about 200 were Islamic preachers."

The Easter Sunday bombings that killed 257 people and wounded nearly 500 were led by a local cleric who travelled to neighbouring India and made contact with militants there.

The minister did not give the nationalities of those who have been expelled, but police have said many foreigners who have overstayed their visas since the Easter attacks were from Bangladesh, India, the Maldives and Pakistan.

"There are religious institutions that have been getting down foreign preachers for decades," Mr Abeywardena said.

"We have no issues with them but there are some that mushroomed recently. We will pay more attention to them."

The country on Sunday marked two weeks since the attacks. Sri Lankan Catholics celebrated Sunday Mass in their homes for a second week as churches remain closed after reports of more possible terrorist attacks.

Cardinal Malcolm Ranjith, the Archbishop of Colombo, offered a televised Mass from his home, which was attended mostly by priests and nuns.

A letter from Pope Francis addressed to him was read out at the end of the service, in which the pontiff said he prayed that "hearts hardened by hatred may yield to his will for peace and reconciliation among all his children".

At St Anthony's Shrine in Colombo, one of the sites hit by the bombings, a Mass was celebrated for a small group of children and youth as a means for inner healing.

Sri Lanka has imposed a state of emergency since the attacks, and given wide powers to troops and police to arrest and detain suspects for long periods.

House-to-house searches are being carried out across the country looking for explosives and extremist propaganda.

Sri Lanka ordered tens of thousands of security personnel to secure public schools before their reopening on Monday.

The wide-scale search for explosives has led to security cordons around 10,900 schools nationwide.

Police spokesman Ruwan Gunasekera said police and soldiers combed school premises and surrounding areas to make sure it was safe for children to go back on Monday.