Guinea-Bissau's ousted former President Kumba Yala occupied the presidential palace for a few hours overnight with a group of armed men, the army says.
Yala was ousted in a coup in 2003
The army announced his move on national radio and issued an appeal for calm, before saying Mr Yala had gone home.
Later there were angry protests against Mr Yala in parts of the capital. Ten days ago he declared himself leader again, two years after the coup.
Elections are due in June to replace the transitional administration
It has ruled the country since Mr Yala's removal.
The impoverished West African nation has been plagued by coups and chaos since independence from Portugal in 1974.
An army statement said: "A group of soldiers took former President Kumba Yala very early this morning to the place so that he could occupy the presidential seat."
He spent about four hours there before leaving, the army said.
The streets are reported to be calm.
The army's chief of staff, Tagme Na Wai, called an emergency meeting at army headquarters, according to a military source.
Mr Yala is believed to have some support in the army, which is dominated by members of his Balanta ethnic group.
He was elected in 200, before being overthrown in 2003.
He was chosen last month by the largest opposition group, the Social Renovation party, as its presidential candidate.
The move was approved by the Supreme Court - despite a five year ban on him taking part in politics.
Regional leaders visited Guinea-Bissau at the weekend to meet Mr Kumba Yala and other leading politicians and top military leaders, in an attempt to calm tensions.
They urged the army to remain neutral and ensure stability ahead of the elections.
They also called on politicians to respect the constitution and the electoral code and warned against acts of violence.