Many anxious Muslims were hunkering down at home but young men, some of them carrying rods, were still zipping around on motorbikes, despite regional curfews from 2 p.m. before the nationwide curfew was imposed.
Sri Lanka widened a curfew on Monday after attacks on mosques and Muslim-owned businesses in the worst unrest since Easter bombings by Islamist militants and blocked Facebook and WhatsApp to stop people inciting violence.
Police said a Catholic priest had sent out a message to parishioners about possible attacks, causing panic among some people in violence-prone areas.
In at least six other towns police and the army used tear-gas to disperse rioters.
Three weeks ago in Sri Lanka extremist bombers blew themselves up in four hotels and three churches, killing more than 250 people. "They have burnt our mosques and smashed many shops owned by Muslims", a resident, who asked not to be identified, told the Reuters news agency.
"Several shops have been attacked", a senior police officer told Agence France-Presse.
There were no immediate reports of casualties or arrests.
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The Muslim shopkeeper's comment was taken by local Christians as a warning of an impending attack.
Al Jazeera's Minelle Fernandez, reporting from Habarana, said the violence was mainly local and sporadic and that authorities wanted to contain the attacks. Christian leaders, including the Sri Lankan archbishop Malcolm Ranjith, have called for restraint from the minority community of about 1.5m people, but Muslim refugees were targeted in small incidents in the week aftermath the attacks, and the tension is now being felt by the wider Muslim community. It has been reported that a mosque and few shops owned by Muslims were attacked by a mob.
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.
"Social media blocked again as a temporary measure to maintain peace in the country", Nalaka Kaluwewa, director general of the government information department, told Reuters on Monday.
This is third time in a month that the Sri Lanka government has banned social media services, all to control the spread of misinformation and rumors online.
Muslims make up almost 10% of Sri Lanka's 22 million people, who are predominantly Sinhalese Buddhists.
On Twitter, Sri Lanka's leading mobile phone operator Dialog said it had also received instructions to block Viber, IMO, Snapchat, Instagram and Youtube until further notice.