Mobs slashed to death a Sri Lankan Muslim man despite a nationwide curfew imposed Monday night after anti-Muslim riots spread to three districts north of the capital Colombo in a violent backlash against Easter suicide bombings.

The 45-year-old man died shortly after admission to a hospital in Puttalam district during the rioting which began Sunday in the area.

"Mobs had attacked him with sharp weapons at his carpentry workshop," a police official in Colombo said. "This is the first death from the riots."

Sri Lankan Prime Minister Ranil Wickremesinghe said the curfew was declared to prevent unidentified groups destabilizing the country by orchestrating communal violence.

"At several places in the North-Western Province these groups created trouble, damaged property," Wickremesinghe said in a televised address to the nation.

"Police and security forces have contained the situation, but these (unidentified) groups are still trying to create trouble."

Wickremesinghe said the unrest would hinder investigations into the April 21 attacks that targeted three Christian churches and three luxury hotels, killing 258 people and wounding nearly 500.

Residents in the North-Western Province were ordered to stay indoors after Christian-led mobs torched dozens of Muslim-owned shops, vehicles and mosques on Sunday and Monday.

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

Later the curfew was extended to cover the entire country of 21 million people.

Police said there were sporadic incidents of mobs throwing stones and torching shops, motorcycles and cars owned by Muslims. In the town of Hettipola, at least three shops were torched.

In the town of Minuwangoda, just north of Colombo, a Muslim-owned hotel and a mosque were attacked by stone-hurling mobs armed with sticks.

In a separate TV address, police chief Chandana Wickramaratne warned police will take stern action against rioters, and constables have been issued orders to use maximum force.

PM warns against rumors on social media

Earlier Monday, authorities banned Facebook, WhatsApp and other social media platforms. Restrictions were also imposed on Google's video platform, YouTube. Twitter was exempted from the ban, considered its small use in the country.

The platforms were blocked to prevent "social unrest via hate messages and false information," Nalaka Kaluwewa, director general of the country's Department of Government Information, told CNN.

The social media ban followed a mob targeting Muslim-owned shops in the town of Chilaw, 80 kilometers north of Colombo on Sunday. The violence broke out because of an argument on Facebook, AFP reported.

"Don't laugh more, one day you will cry," wrote the Facebook post, which local Christians took to be a warning of an impending attack.

The group smashed the man's shop and vandalized a nearby mosque prompting security forces to fire in the air to disperse the crowd, but the violence spread.

Kaluwewa told CNN the ban was "not because of [a] single incident" but had been decided after considering the overall security situation.

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

The prime minister urged the public not to believe rumors and warned that civil unrest will 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 Monday, adding the civil unrest increase the burden of security forces and hamper ongoing investigations

(With inputs from AFP)