The declaration by authorities was met with mixed emotions, the memory of the virus that has left more than 2 500 people dead still too fresh.
The World Health Organisation declared the situation "serious" when new cases surfaced after officials thought they had contained the virus.
Mining firms have locked down operations in Guinea and pulled out some international staff as the death toll from suspected Ebola cases hit 83.
Guinea on Wednesday postponed a presidential election run-off due in four days, casting doubt on the country's bid to return to civilian rule.
Guinea's junta has pledged to pave the way for a return to civilian rule and said that leader Moussa Dadis Camara would need time to recover.
Guinea's ruling military junta, facing rising international pressure, has lifted its ban on public demonstrations.
Mutinous Guinean soldiers launched a coup attempt on Tuesday hours after the death was announced of long-serving President Lansana Conte.
Sierra Leone's President Ernest Bai Koroma on Friday embarked on his first foreign trip since taking office this week, heading to neighbouring Guinea and Liberia to promote ties damaged by more than a decade of war. The former insurance executive was sworn in on Monday within hours of being declared winner of a run-off election.
Guinea will start talks with the International Atomic Energy Agency on a nuclear energy programme after the discovery of uranium this month, becoming the latest African state seeking a nuclear solution to power shortages. The poor West African nation announced at the start of August that Australian miner Murchison United had discovered commercially viable deposits of uranium.