Birtukan Medeksa was among more than 100 people jailed for offences after controversial polls in 2005.
A dispute over the terms of her release caused her re-arrest.
Her colleague Berhanu Nega, who was also pardoned and now lives in exile, told the BBC it showed the government “was hell-bent on staying in power”.
The BBC’s Elizabeth Blunt in the capital, Addis Ababa, says Ms Birtukan was a former judge and one of the younger and more charismatic leaders of the coalition which did so astonishingly well against the ruling party in the 2005 elections.
Our reporter says that while in jail facing charges of treason, she became even more of a heroine, attracting widespread sympathy as a single mother separated from her baby daughter.
After the opposition leaders were pardoned and released last year, she emerged as the leader of a new coalition, the Union for Democracy and Justice (UDJ), painstakingly stitched together from various opposition groupings to contest elections in 2010.
The government news agency, quoting the ministry of justice, said her pardon had been revoked, since she had denied requesting her pardon.
Ms Birtukan’s problems started when she spoke to journalists abroad about the way the opposition leaders were released, our correspondent says.
She talked about negotiations which had taken place between the opposition and government, with the help of a panel of elders, before their pardon was granted.
The government prefers to lay emphasis on a document signed by the prisoners, regretting any mistakes they had committed and asking for pardon.
This implies that their release was part of a normal judicial process, rather than in any way part of a negotiated political deal.
Mr Berhanu, who was elected mayor of Addis Ababa in 2005 and now lives in the United States, says the aim of the government is to “humiliate” any opposition to its rule.
“What this is doing is to change the nature of the struggle,” he told the BBC’s Network Africa programme.
“There is no other way to freedom and liberty in that country without getting rid of this government.”
Story from: BBC NEWS