Critics accuse Meles Zenawi of suppressing his opponents
Ethiopia's former President Negasso Gidada has joined an opposition party, as the country builds up to an election scheduled for next May.
Mr Negasso, in power between 1995 and 2001, said he had joined the Unity for Democracy and Justice Party (UDJ) to try to unite Ethiopia.
Analysts say his defection and that of ex-Defence Minister Seye Abraha are likely to boost the UDJ's popularity.
Its leader is in jail after protests following the last election in 2005.
Birtukan Medeksa was arrested after violence broke out when opposition parties organised protests, citing election fraud.
Some rights groups have accused Prime Minister Meles Zenawi of trying to ensure election victory by suppressing opposition - allegations he denies.
The BBC's Uduak Amimo in Addis Ababa says the two defections are a significant symbol of opposition to the government.
But she says the UDJ and its allies are unlikely to overhaul the governing party in next year's election.
Mr Negasso, whose role as president was largely symbolic, is said to be a popular politician.
He told Reuters news agency: "Our joining the UDJ sends a signal that we have to work hard for the unity of the country and the Ethiopian people."
Some 200 people were killed after security forces opened fire during the protests which followed the 2005 elections.
More than 100 opposition leaders, activists and journalists were convicted and jailed but most have since been pardoned.
Ms Birtukan, 36, was freed but then re-arrested and sentenced to life in prison after criticising the terms of the amnesty.
Analysts say the detentions have seriously weakened the opposition.
The UDJ has been stitched together from several different formations.