A decree signed by Guinea president Alpha Conde said the government had decided to requisition all personnel, equipment, facilities, equipment and assets of the company from May 2, until the dispute is settled.
“The Post and Telecommunications Regulatory Authority will provisionally administer the company until the final resolution of the dispute,” the decree, which had no date, said.
MTN said it was negotiating with the government.
“We are confident that both parties will reach an amicable solution that will be in the best interest of all the stakeholders affected, including the millions of MTN subscribers in Guinea,” Rich Mkhondo, corporate affairs executive at MTN Group said.
Last week the Guinean government warned that it would suspend the local licence of MTN if a row over payments due to it after MTN's 2007 acquisition of a local operator were not resolved by Friday.
MTN operates in Guinea under the Areeba brand.
The row with MTN is the latest in a series of showdowns the government has had with international firms since Conde took over as president following two years of military rule.
MTN entered the Guinean market when it bought the Areeba brand from Lebanese operator Investcom in 2007. But the Guinean government says it is still owed $22.25m from the deal.
MTN, which says it has two million subscribers in Guinea, is the leading mobile operator in the West African nation, where the population is estimated at 10 million.
Since winning an election last year, Conde has cancelled a port deal with France's Getma international, suspended a railway project with Brazil’s Vale, and his camp has signalled more tough moves ahead, possibly against Russian miner RUSAL.