Abbraar Masjid mosque is seen after a mob attack in Kiniyama, Sri Lanka. Picture: Dinuka Liyanawatte/Reuters

Colombo - Sri Lanka has imposed a nationwide curfew from 9 pm (1530 GMT) to 4 am the following day as a precautionary measure to prevent a flare-up of ethnic and religious violence in the wake of the deadly Easter Sunday attacks, police said on Monday.

Police said sporadic incidents were first reported from the North Western Province where a curfew was imposed from Monday afternoon to prevent the spread of violence.

By night the curfew was extended to other areas in the country.

Prime Minister Ranil Wickremesinghe in a statement said any ethnic violence could cost security and police resources that should be used for investigations regarding the Easter Sunday bombings.

"I have ordered the forces to strictly enforce the curfew and take action against those creating violence," he said.

He said that the country would be destabilized if the attacks result in ethnic violence.

The tension started rising Sunday after an incident between the Muslim minority and the Sinhalese majority due to inflammatory messages on social media.

A message shared on Facebook prompted several people to surround and damage an Islamic religious centre on Sunday in the town of Chilaw, 80 kilometres north of the capital Colombo.

Police arrested one person, identified as Abdul Hameed Mohamed, who had allegedly posted an inflammatory message on Facebook.

On Monday, some social media networks were temporarily blocked in Sri Lanka after messages shared online raised religious tensions between some Muslims and Sinhalese.

Police spokesman Ruwan Gunasekara said there were other false messages on social media which claimed a group of terrorists was arrested close to the capital and that three people pretending to be medical students were arrested at a hospital on the city's outskirts.

Armed forces and police increased security on Monday in the wake of reports that a terrorist group was planning to carry out attacks in several locations in the capital.

Attendance at schools and offices dropped, mostly in Colombo and larger towns.

Security has been tight in Sri Lanka since Easter Sunday, when 257 people were killed in suicide attacks in three churches, three luxury hotels and two other locations. The bombers were identified members of an Islamist extremist group.

Meanwhile, Sri Lankan President Maithripala Sirisena departed for China on Monday and appointed Ruwan Wijewardene as the acting defence minister in his absence. The Easter Sunday bombings occurred while Sirisena, who also serves as defence minister, was on a private trip to Singapore, and he was criticized for failing to name an acting defence minister during his absence.