Sri Lanka said on Wednesday that hardline Buddhist groups were likely to blame for a wave of anti-Muslim riots that swept the island this week in apparent retaliation for Easter bombings that were claimed by the militant Islamic State group.

The April 21 attacks targeted churches and hotels, killing more than 250 people and fuelling fears of a backlash against the nation's minority Muslims.

In the anti-Muslim unrest that started on Sunday, mobs moved through towns in Sri Lanka's northwest, ransacking mosques, burning copies of the Holy Quran and attacking shops with petrol bombs, residents said.

Authorities have arrested some 78 suspected rioters, including three described as Sinhala Buddhist extremists who had been investigated for similar actions in the town in Kandy district last year.

“These are organised attacks on Muslim business houses and premises,” Navin Dissanayake, minister of plantation industries, said during a government news conference about the security situation.

Asked who was organising the attacks, Dissanayake said: “I think these organisations that Amith Weerasinghe, Dan Priyasad, and Namal Kumara (are heading),” referring to the three Buddhist extremists arrested on Tuesday.

Local media reported that Priyasad was released on bail on Wednesday while Weerasinghe was remanded until May 28. The status of Kumara was not clear.

A police spokesperson was not immediately available for comment on the arrests.

Muslims make up nearly 10 per cent of Sri Lanka's population of 22 million, which is predominantly Buddhist. The Indian Ocean island was torn for decades by a civil war between separatists from the mostly Hindu Tamil minority and the Sinhala Buddhist-dominated government. The government stamped out the rebellion about 10 years ago.

In recent years, Buddhist hardliners, led by the Bodu Bala Sena (BBS) or “Buddhist Power Force” have stoked hostility against Muslims, saying Middle Eastern influence has turned the community more conservative and insular.

In the same press conference, Ranjith Madduma Bandara, minister of public administration, said the group behind the attacks had political aims.

“This group is trying to tarnish the government's image and show the government is unable to handle the situation,” he said, without naming the organisation.

Fear persists

Authorities said the island was calm again, with no anti-Muslim violence reported on Wednesday. But Muslims in NWP remained nervous and stayed indoors on Wednesday, after sword-wielding rioters killed one man late on Monday while vandalising scores of shops and mosques.

In Bingiriya, where some 2,000 people went on the rampage, Muslim cleric M.I.M. Siddeeque said the community was worried.

“Our people are still afraid to go out,” he told AFP by telephone.

In over a dozen interviews in the hard-hit Kurunegala district northeast of Colombo, Muslims said attacks took place despite the presence of security forces. Two residents of Thunmodara, a town to the northeast of the capital Colombo, told Reuters that a mosque and some Muslim-owned shops were attacked.

Despite the extraordinary security measures, the minorities felt insecure, said Dissanayake, who is also a government minister.

“People feel insecure and I acknowledge that,” Dissanayake told reporters in Colombo. "We have given the armed forces of this country... a complete free hand to tackle the security situation."

One police source who declined to be identified told Reuters they did not have enough officers to handle the rioters. Police spokesman Ruwan Gunasekera on Tuesday rejected allegations that police had stood by.

But the police have made several changes in the command structure by removing senior officers in the troubled areas after allegations officials did nothing to stop the rioters.

Meanwhile, the military released CCTV footage which showed a man in uniform apparently standing by while a mob pelted stones at a home, and sought public help to identify the individual.

“The attention of the army has been drawn to a video clip where a person dressed in uniform similar to that of the army was watching while a group of violent saboteurs were in action in the general area of Thunmodara,” the army said in a statement announcing the investigation.

Parliamentary speaker Karu Jayasuriya condemned the violence and appealed for calm.

"The attacks on mosques, the burning of houses and shops, the attacks on innocent people cannot be condoned in any way," Jayasuriya said in a statement.

The attacks came during the Muslim holy fasting month of Ramazan.

A state of emergency has been in place since the Easter bombings and security forces have been given sweeping powers to detain suspects.

Internet service providers said the telecoms regulator on Tuesday extended a social media ban to Twitter.

Earlier, Facebook, WhatsApp, YouTube and Instagram had been blocked to prevent the spread of messages inciting violence.