“The Criminal Investigation Division of police today arrested Nalaka De Silva after five days of questioning him,” police spokesman Ruwan Gunasekara said.
He added that Silva was accused of plotting to kill the president, several senior police officials and a former defence secretary.
New Dehli and Colombo have both denied the claim.
Silva will be held until November 7, on magistrates’ orders. He was arrested after a police informant made the allegations against Silva in a news conference.
Investigators say they are seeking access to deleted data from the informant’s mobile phone with the help of Chinese telecommunications company Huawei after a court granted permission to request the company’s help.
A Huawei spokesman told Reuters news agency that the company had received no such request from Sri Lanka and Silva’s lawyers were not immediately available for comment.