Sri Lankan authorities have either killed or arrested all the militants responsible for the Easter Sunday bombings that left 257 people dead, police chief Chandana Wickramaratne said on Tuesday.

Police have accounted for every individual involved in the April 21 attacks on three churches and three luxury hotels, Wickramaratne said in an audio statement.

"All those who organised and carried out the suicide bomb attack have died or is in our custody," he said.

"The two bomb experts of the group have been killed. We have seized the explosives they had stored for future attacks."

Wickramaratne, who was named acting police chief last week after President Maithripala Sirisena suspended his predecessor over his failure to act on warnings about the attacks, said public life was slowly returning to normal with the lifting of curfews imposed after the bombs.

The government reopened public schools on Monday, but attendance dropped to below 10 per cent in many places with parents still fearing attacks.

"We have strengthened security for all schools," the police chief said. "We are also conducting a programme to create awareness about safety and security in all schools."

Wickramaratne did not say how many people were in custody over the bombings, but police spokesman Ruwan Gunasekera said on Monday that 73 people, including nine women, were being held.

The bombings were blamed on a local group, the National Thowheeth Jamaath (NTJ), but the militant Islamic State group has also claimed responsibility.

Police said religious tensions have eased at Negombo, a town north of Colombo that suffered the highest death toll in the Easter Sunday attacks. A bomb at St Sebastian's church in the town killed more than 100 worshippers.

Dozens of Muslim-owned businesses, homes and vehicles in Negombo were damaged in clashes on Sunday night.

The Roman Catholic Church appealed for calm and urged Christians not to carry our revenge attacks against Muslims.

Police said two arrests were made and more suspects had been identified through CCTV footage.