Mr Nour made an unprecedented bid to replace the long-serving Hosni Mubarak
Egyptian authorities have released the country's best known opposition figure, Ayman Nour, after three years in jail.
Judiciary officials said Mr Nour, 44, was released on health grounds.
Speaking by telephone from his Cairo home, Mr Nour said he would resume leadership of his Ghad (Tomorrow) party and rejoin political life.
Mr Nour came a distant second in Egypt's first contested presidential poll in 2005 but was jailed for alleged election fraud in the following months.
The US, Egypt's close ally and major aid donor, took great interest in the Nour case and described his conviction as a miscarriage of justice.
The 44-year-old lawyer is known to suffer from diabetes and is dependent on insulin injections.
Speaking to reporters by telephone from his home, Mr Nour thanked God that he had been released.
"I am going to practise my role as a politician through the Ghad party and through my previous role," he added.
While in jail, Mr Nour believed the regime wanted him to die behind bars, he told the AFP news agency during an interview last year.
He described himself as having gone from a victim of "political assassination" to being subjected to "physical destruction".
Mr Nour formed his political party in 2004 with a view to contesting the presidential election the following year, run as a multi-candidate affair for the first time after US put pressure on Egypt to introduce democratic reforms.
He came a distant second in the poll, which he claimed was rigged, winning 8% of the vote against the 89% polled by the incumbent Hosni Mubarak.
Three months after elections, Mr Nour was charged with forging signatures to register his party and jailed for five years.
Analysts at the time said the speed with which Ayman Nour was stripped of parliamentary immunity and brought to trial suggested the government did underestimate the political threat he posed.
The government of Mr Mubarak, who has ruled without interruption since 1981, and the judiciary have dismissed allegations that the trial was politically motivated.
Washington had repeatedly called for his release, although correspondents say US criticism of the case has died off in recent months, having been a cause of heightened tension in the past.