Sri Lanka imposed a nationwide six-hour night curfew and banned Facebook, WhatsApp and other social media platforms on Monday after anti-Muslim riots gripped several towns in the latest fallout from the Easter bombings.

Christian groups attacked Muslim-owned shops in a sign of the ongoing religious tension in Sri Lanka since the April 21 attacks by suicide bombers on three hotels and three churches which left 258 dead.

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A Muslim man was slashed to death, making him the first fatality in the riots, police said. The 45-year-old man died shortly after admission to a hospital in Puttalam district, a police official told AFP.

A night curfew in several towns north of Colombo was lifted at dawn, but reimposed 10 hours later as tensions were fuelled by persistent rumours of mob violence. The curfew was later extended across the island, police said in a statement.

Police said a Catholic priest had sent out a message to parishioners about possible attacks, causing panic among some people in violence-prone areas.

Prime Minister Ranil Wickremesinghe urged the public not to believe rumours and warned that civil unrest will only stretch the already thinly deployed security forces.

“I appeal to all citizens to remain calm and not be swayed by false information,” Wickremesinghe said on Twitter, which was not targeted in the social media blockade.

“Security forces are working tirelessly to apprehend terrorists and ensure the security of the country, but each time there is civil unrest, we increase their burden and hamper ongoing investigations.”

A state of emergency has been in place since the bombings — which the militant Islamic State (IS) group claims to have helped — and security forces have been given sweeping powers to arrest and detain suspects for long periods.

Police said a mob targeted shops in the northwestern town of Chilaw on Sunday in anger at a Facebook post by a shopkeeper. Security forces fired into the air to disperse the crowd, but the violence spread to nearby towns where Muslim businesses were also attacked.

A motorcycle gang attacked shops in nearby Kuliyapitiya and four members were arrested, officials said.

However, dozens of people laid siege to the police station and forced their release.

Despite a night curfew, a mosque was vandalised, local residents said.

Mosque attack

“Don't laugh more, one day u will cry,” was posted on Facebook by a Muslim shopkeeper, and local Christians took it to be a warning of an impending attack.

Mobs smashed the man's shop and vandalised a nearby mosque prompting security forces to fire in the air to disperse the crowd. A curfew was imposed from Sunday afternoon until dawn Monday.

There have already been clashes between Christians and Muslims in Negombo, the town north of Colombo that was targeted by the suicide attackers.

The main body of Islamic clerics, the All Ceylon Jamiyyathul Ulama (ACJU), said there was increased suspicion of Muslims after the Easter Sunday attacks.

“We call upon the members of the Muslim communities to be more patient and guard your actions and avoid unnecessary postings or hosting on social media,” the ACJU said.

Internet service providers said they have been instructed by the telecommunications regulator to block access to Facebook, WhatsApp, YouTube and Instagram.

The latest unrest came as Catholic churches resumed public Sunday masses for the first time since the bombings. Worshippers were searched before being allowed into churches that were guarded by armed police and troops. There were no reports of disruption to services, however.

Schools reopen

Dozens of people have been detained since the Easter Sunday attacks, and amid the heightened security, police have banned parking near schools and students are allowed in after checking for explosives.

Public schools completed their reopening from extended Easter holidays after the attacks, but attendance was extremely low, according to education authorities.

Private Catholic schools were to open on Tuesday, but many were planning to postpone the reopening until next week, parent groups said.

Muslims make up around 10 per cent of Buddhist-majority Sri Lanka's 21 million population and Christians about 7.6pc.