Sri Lankan President Maithripala Sirisena says police are looking for 140 people believed to have links with the Islamic State group, which has claimed responsibility for Easter Sunday suicide bombings.
Muslims in Sri Lanka were urged to pray at home on Friday and not attend mosques or churches after the State Intelligence Services warned of possible car bomb attacks, amid fears of retaliatory violence for the bombings that killed 253 people, revised down on Friday from 359.
The archbishop of Colombo, Cardinal Malcolm Ranjith, also said on Friday there would be no Sunday Masses until further notice. He appealed for support to rebuild the lives of affected people and reconstruct churches.
Nearly 10,000 soldiers were being deployed across the Indian Ocean state to carry out searches and provide security for religious centres, the military said.
Fears of retaliatory sectarian violence have already caused Muslim communities to flee their homes amid bomb scares and lockdowns.
The All Ceylon Jamiyathul Ullama, Sri Lanka's main Islamic religious body, encouraged Muslims to conduct prayers at home on Friday in case "there is a need to protect family and properties".
Police have detained at least 76 people to date, including foreigners from Syria and Egypt. Authorities also confirmed the suspected leader of the attacks died in the Shangri-La hotel bombing.
Islamic State provided no evidence to support its claim it was behind the attacks. If true, it would be one of the worst attacks carried out by the group outside Iraq and Syria.
The extremist group released a video on Tuesday showing eight men, all but one with their faces covered, standing under a black Islamic State flag and declaring their loyalty to leader Abu Bakr Al-Baghdadi.
The government said nine homegrown, well-educated suicide bombers carried out the attacks, eight of whom had been identified. One was a woman.
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Sirisena said some Sri Lankan youths had been involved with Islamic State since 2013, and said information uncovered so far suggested there were 140 people in Sri Lanka involved in Islamic State activities.
"Police are looking to arrest them," Sirisena said.
Authorities have so far focused their investigations on international links to two domestic Islamist groups - National Thawheed Jama'ut and Jammiyathul Millathu Ibrahim - they believe carried out the attacks.
Government officials have acknowledged a major lapse in not widely sharing an intelligence warning from India before the attacks.
Sirisena said top defence and police chiefs had not shared information with him about the impending attacks. Defence Secretary Hemasiri Fernando resigned over the failure to prevent the attacks.
He also blamed Prime Minister Ranil Wickremesinghe's government for weakening the intelligence system by focusing on the prosecution of military officers over alleged war crimes during a civil war with Tamil separatists.
The Easter Sunday bombings shattered the relative calm that had existed in Buddhist-majority Sri Lanka since a civil war against mostly Hindu ethnic Tamil separatists ended 10 years ago.
Sri Lanka's 22 million people include minority Christians, Muslims and Hindus. Until now, Christians had largely managed to avoid the worst of the island's conflict and communal tensions.