The African Union has denounced the appointment of a son of Togo President Gnassingbe Eyadema to succeed his father hours after he died on Saturday.
Eyadema rose to power in a military coup
AU leaders described the naming of Faure Eyadema as a military coup.
The army announced the replacement saying the speaker of parliament who should have taken over under the constitution was out of the country.
It is reported he was unable to get in because the West African country's borders have been sealed by the army.
Army Chief of Staff Gen Zakari Nandja said the decision had been taken to avoid a power vacuum.
Faure Eyadema is the current communications minister.
Eyadema, 69 and Africa's longest-serving ruler, died while being evacuated for medical treatment abroad. Officials said he had suffered a heart attack.
The plane bringing the speaker of parliament Fambare Natchaba Ouattara back home from Paris was reportedly not allowed in because of the closures and had to proceed to Benin.
Eyadema had ruled since 1967, when he rose to power in a military coup.
The announcement of his death was made by Prime Minister Koffi Sama.
"It is a national catastrophe. The president of the republic, Gnassingbe Eyadema, is no more," he said.
"The government, the armed forces and security forces will keep watch so that order, security and peace reign throughout the country."
The handover of power to Faure Eyadema, 39, was announced on national television by Gen Zakari Nandja.
"The armed forces (FAT) have been confronted with the fact that there is a total power vacuum," he said.
African Union commissioner Alpha Oumar Konare said his organisation would not support a leader who had not been elected.
"What's happening in Togo needs to be called by its name: it's a seizure of power by the military, it's a military coup d'etat," Mr Konare told AFP news agency.
AU chairman - Nigerian President Olusegun Obasanjo - called Eyadema "one of Africa's greatest leaders", but added that that the AU would not accept any unconstitutional transition of power in Togo, his spokesman said.
Eyadema had travelled abroad on several occasions in recent years for medical treatment.
After seizing power 38 years ago, Eyadema dissolved all political parties and governed unchallenged for more than two decades.
He legalised political parties in 1991, as a result of popular pressure, and won three elections.
But accusations of political repression and electoral fraud continued.
The European Union suspended aid in 1993 in protest at alleged voting irregularities, although some ties have since been restored.
An investigation by the United Nations and the Organisation of African Unity, after disputed elections in 1998, concluded there had been systematic violations of human rights.
Recently, Eyadema had tried to change his political image by appearing more moderate and attempting to involve himself in peace moves elsewhere in the region.
However, these attempts were largely unsuccessful, says the BBC's West Africa correspondent Andrew Simmons.