Omrie Golley was "arrested and detained after suspicion he was engaged in subversive activities," Ahmed Tejan Kabbah said in an announcement broadcast on state radio and television on Thursday.
The government was "treating the matter very seriously", President Kabbah said.
Golley was a spokesman and senior political leader for the Revolutionary United Front rebel group, whose fighters infamously severed the limbs of their victims, terrorising the country and killing tens of thousands of civilians during a decade-long war that began in 1991.
The rebellion was crushed by a combination of British troops, UN forces and soldiers from neighbouring Guinea, and Sierra Leone has been mostly peaceful since.
The UN mission pulled out of
Sierra Leone at the end of 2005
A 17,000-strong UN mission, once the largest in the world, pulled out of the country in the final days of 2005.
Golley is believed to have been living in exile in London for the last few years, and returned to the country two weeks ago.