Before his imprisonment, Ayman Nour was a relative newcomer to Egypt's stagnant political scene.
Mr Nour, a softly-spoken, eloquent former lawyer, formed his political party in October 2004 with a view to contesting presidential elections the following year.
Ayman Nour's Ghad party was founded in October 2004
Three months later, prosecutors in Cairo charged him with forging signatures to register Ghad, the party whose name means "tomorrow" in Arabic.
He developed a vocal band of supporters at home and a profile abroad.
And his liberal credentials brought him into direct competition with the youthful wing of the governing party, headed by President Hosni Mubarak's son, Gamal.
They also brought him to the attention of the US, Egypt's biggest ally in the West, which has urged Cairo to reform the political system that has kept President Mubarak in power for more than two decades.
Analysts said the speed with which Mr Nour was stripped of parliamentary immunity and brought to trial suggested the government did not want to under-estimate the political threat he posed.
The government rejected all allegations that the trial was politically motivated.
Washington voiced disquiet at Mr Nour's treatment and Cairo delayed his trial, enabling him to take part in the 2005 elections.
The presidential poll saw Mr Nour come a distant second to the incumbent, polling 8% of the vote to Mr Mubarak's 89% - a result Nour alleged was rigged.
In November 2005, Mr Nour also lost his parliamentary seat to a ruling party candidate - another result that he claimed was rigged.
His trial went ahead a month later, delivering a guilty verdict and handing him a five-year jail term.
A co-defendant at the trial complained he had been forced to make a false confession.
Mr Nour, a diabetic dependent on insulin, spent the week before the verdict in hospital as a result of a hunger strike he had started in protest at his detention.
Ayman Nour's wife has campaigned for his release
During an earlier spell in prison, he wrote to US magazine Newsweek, saying the government was suspicious of his reformist inclinations and wanted to discredit him by labelling him as an agent of the US.
"The solidarity shown to me by my supporters, together with sympathy from the international community, have triggered in [the] authorities a strange stubbornness," he wrote.
The 44-year-old comes from a family with a long history of involvement in public life.
Throughout his trial, his wife, Gameela Ismail, led daily protests against the Mubarak administration.
While in prison last year, he is known to have written to Barack Obama as he campaigned for the US presidency. It is understood he urged Mr Obama to help Arab reformers push for democracy in the Middle East.
Meanwhile, during a speech in Egypt in May 2008, President George W Bush pointedly remarked that "too often in the Middle East, politics consisted of one leader in power and the opposition in jail".
Mr Nour's release was unexpected but comes at a time of expectation that the Obama administration could bring a change in diplomatic relations in the region.