Violence also occured in other parts of the country last week.
"I cannot stand by while ill-motivated people drag this country once more towards a state of anarchy," Koroma said.
Fears of war
Sierra Leone is struggling to recover from the brutal 1991-2001 civil war which saw about 120,000 people killed and thousands mutilated and raped by rival forces.
The violence of the last days has raised fears among many residents that the west African nation could be plunged into chaos once more.
"The APC believes that a change of government is only legitimate when it is made through the ballot box"
President of Sierra Leone
"The stability of the nation remains my utmost priority and I will stop at nothing to defend the life and property of the people," Koroma said.
"I am aware of the machinations of certain individuals who are bent on making the nation ungovernable to discredit the effort of the government," he added, without naming names.
The president said he had instructed the authorities to undertake "a careful and thorough investigation" into the clashes in the southern Pujehun area, southeastern Kenema and Freetown.
He lashed out at the SLPP - which was in power between 2002 and 2007 - insisting that his APC party never tried to disrupt the SLPP leadership when they were in opposition.
"The APC believes that a change of government is only legitimate when it is made through the ballot box," Koroma said.
On Saturday, authorities banned an APC and a SLPP radio station accused of fanning the tensions.