A former Liberian army commander has been arrested for "subversive activities" - the first such arrests since landmark elections in 2005.
Ellen Johnson-Sirleaf took power last year after winning elections
Gen Charles Julu headed the presidential guard under former leader Samuel Doe and led a 1994 coup attempt.
"There is hard evidence that this man was trying to plan a coup," Information Minister Laurence Bropleh told the BBC.
President Ellen Johnson-Sirleaf took power in 2006 after a 14-year war.
No formal charges have been made over the allegations.
But the BBC's Jonathan Paye-Layleh in the capital, Monrovia, says people are shocked at the possibility of renewed unrest in the country.
The United Nations has some 15,000 peacekeepers in Liberia - the second largest deployment in the world.
Mr Bropleh said there was video proof of the coup pot, which the authorities in neighbouring Ivory Coast had helped investigate.
Fighters from the two countries have been involved in conflicts on both sides of the border in recent years.
"The Liberian public should remain calm. There is no immediate threat to the state," he told Reuters news agency.
Student groups from Doe's Krahn ethnic group have condemned the arrests and called for the release of Gen Julu and the other former officer also arrested.