"The president has declared an emergency," Fernando said in the early hours of Saturday, speaking hours after Foreign Minister Lakshman Kadirgamar died in a Colombo hospital after being shot in the head and chest by snipers.
Kadirgamar was also a close confidante of President Kumaratunga.
The state of emergency would last for an indefinite period, said Fernando. It allows security forces to arrest and detain suspects for lengthy periods.
The announcement of the state of emergency came as the military asked Colombo residents to remain indoors on Saturday to allow a major search operation to go ahead for two suspected Tiger rebels behind the killing of Kadirgamar.