COLOMBO (Reuters) - Sri Lankan police are trying to track down 140 people believed linked to Islamic State, which claimed responsibility for the Easter Sunday suicide bombings that killed 253, as shooting erupted in the east during a raid.

Muslims in Sri Lanka were urged to pray at home after the State Intelligence Services warned of possible car bomb attacks, amid fears of retaliatory violence.

And the U.S. Embassy in Sri Lanka urged its citizens to avoid places of worship over the weekend after authorities reported there could be more attacks targeting religious centers.

Archbishop of Colombo Cardinal Malcolm Ranjith told reporters he had seen a leaked internal security document warning of further attacks on churches and there would be no Catholic masses this Sunday anywhere on the island.

The streets of Colombo were deserted on Friday evening, with many people leaving offices early amid tight security after the suicide bombing attacks on three churches and four hotels that also wounded about 500 people.

(GRAPHIC: Sri Lanka bombings - tmsnrt.rs/2Xy02BA)

President Maithripala Sirisena told reporters some Sri Lankan youths had been involved with Islamic State since 2013. He 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.

Nearly 10,000 soldiers were deployed across the Indian Ocean island state to carry out searches and provide security for religious centers, the military said on Friday.

The All Ceylon Jamiyathul Ullama, Sri Lanka’s main Islamic religious body, urged Muslims to conduct prayers at home in case “there is a need to protect family and properties”.

Illustrating the tension that has gripped the country, shooting erupted between security forces and a group of men in the east during a search and cordon operation, a military spokesman said.

The raid took place in the town of Ampara Sainthamaruthu near Batticaloa. The spokesman said there was an explosion in the area and when soldiers went to investigate they were fired upon. No details of casualties were immediately available.

Police have detained at least 76 people, including foreigners from Syria and Egypt, in their investigations so far.

Islamic State provided no evidence to back its claim that 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 its leader, Abu Bakr Al-Baghdadi.

DEFENSE, POLICE CHIEFS QUIT

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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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.

(GRAPHIC: A decade of peace shattered - tmsnrt.rs/2W4wZoU)

Sirisena said top defense and police chiefs had not shared information with him about the impending attacks. Defense Secretary Hemasiri Fernando resigned over the failure to prevent the attacks.

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“The police chief said he will resign now,” Sirisena said.

He 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 decade-long civil war with Tamil separatists that ended in 2009.

Sirisena fired Wickremesinghe in October over political differences, only to reinstate him weeks later under pressure from the Supreme Court.

Opposing factions aligned to Wickremesinghe and Sirisena have often refused to communicate with each other and blame any setbacks on their opponents, government sources say.

Cardinal Ranjith said that the church had been kept in the dark about intelligence warning of attacks.

“We didn’t know anything. It came as a thunderbolt for us,” he said.

The Easter Sunday bombings shattered the relative calm that had existed in Buddhist-majority Sri Lanka since the civil war against mostly Hindu ethnic Tamil separatists ended.

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.

Most of the victims were Sri Lankans, although authorities said at least 38 foreigners were also killed, many of them tourists sitting down to breakfast at top-end hotels when the bombers struck.

They included British, U.S., Australian, Turkish, Indian, Chinese, Danish, Dutch and Portuguese nationals. Britain warned its nationals on Thursday to avoid Sri Lanka unless it was absolutely necessary.

Fears of retaliatory sectarian violence have already caused Muslim communities to flee their homes amid bomb scares, lockdowns and security sweeps.

But at the Kollupitiya Jumma Masjid mosque, tucked away in a Colombo side street, hundreds attended a service they say was focused on a call for people of all religions to help return peace to Sri Lanka.

“It’s a very sad situation,” said 28-year-old sales worker Raees Ulhaq, as soldiers hurried on dawdling worshippers and sniffer dogs nosed their way through pot-holed lanes.

“We work with Christians, Buddhists, Hindus. It has been a threat for all of us because of what these few people have done to this beautiful country.”

(This story has been refiled to remove extraneous words from paragraph two.)