A powerful explosion blew up a pipeline supplying water to parts of the capital and immediate suburbs, police said on Thursday.
There was no immediate claim of responsibility for the blast at Boralesgamuwa, just south of Colombo, late on Wednesday. There were no casualties or damage to other property, police said.
Water supply to tens of thousands of homes was stopped because of the sabotage, police said.
The attack came hours after the government slapped new anti-terror laws aimed at Tamil Tiger guerrillas who were accused of trying to assassinate a top defence official last week.