President Tandja was deeply unpopular before the coup
At least 10 former ministers and officials loyal to ex-President Mamadou Tandja have been arrested in Niamey.
Among those detained were the former ministers of finance and justice, and the heads of the national electricity and water companies.
Interior Minister Cisse Ousmane said they were accused of carrying out subversive activities.
In February, Mr Tandja was overthrown in a coup and replaced by a military junta led by Maj Salou Djibo.
The junta, which calls itself the Supreme Council for the Restoration of Democracy, has said it wants to make Niger an example of "democracy and good governance".
After the coup, the largely arid but uranium-rich country was expelled from the African Union and the international community urged the junta to hold elections as soon as possible.
Mr Tandja had ruled Niger for more than a decade before the February coup and had come under increasing criticism both at home and abroad.
In 2009, opposition parties accused Mr Tandja of effectively mounting a coup when he forced through a change in the constitution to secure an unprecedented third term as president.