Kizza Besigye in profile
Kizza Besigye used to be President Yoweri Museveni's personal doctor but he now refers to Uganda's leader as a "dictator".
The two men have not spoken to each other for 10 years.
Dr Besigye has lost three disputed elections to President Museveni - the most recent in February. The opposition leader has said all the votes were flawed.
Before the 2011 polls, Dr Besigye had called for Egypt-style uprisings in the event of fraud.
And after gaining 26% to the president's 68% in the vote, the leader of the Forum for Democratic Change (FDC) claimed it was rigged.
Since the election the opposition has raised its profile by attempting to stage a series of "walk-to-work" protests over the rising cost of living. In being blocked by the authorities, they have exposed the brutality of the police and military.
Dr Besigye has been arrested repeatedly. On one occasion he was shot in the hand, on another he suffered eye injuries after being doused in pepper spray. He travelled to Kenya for treatment.
The authorities have said Dr Besigye is provoking them, and he has been charged with inciting violence.Slapped
The charismatic Dr Besigye, 54, is married to Winnie Byanyima, a former MP who is seen as intelligent and ambitious and who was once a close personal friend of Mr Museveni. Dr Besigye and his wife have one son.
He was born in the western Rukungiri District, the second child in a family of six and the son of a policeman.
Besigye's dates with history
- 1956: Born
- 1975: Went to Kampala
- 1979: Joins Museveni's movement
- 1981: Imprisoned
- 1982: Becomes Museveni's doctor
- 1986: Named minister
- 2001: Challenges Museveni in elections. Flees Uganda after losing
- 2006: Charged with treason and rape before election - later acquitted on both counts
Both his parents died while he was at secondary school.
In 1975, he headed to Kampala and to medical school at what was then the region's most prestigious university, Makerere.
Idi Amin was president of Uganda at the time and his army was causing misery in many areas of the country, including on the university campus.
Mr Besigye did not escape that treatment. Aged 18, he was in a Kampala hotel about to have dinner. Walking to the toilets, he stopped to talk to a former class mate.
A huge man lifted him up by the collar, slapped him hard across the face and sent him sprawling to the floor.
He said he never made it to the toilets, and never ate his dinner. He picked himself up and ran for his life.
Following the overthrow of Idi Amin, Dr Besigye - as he now was - became a member of the Uganda People's Movement of Yoweri Museveni.
The movement had little success in the 1980 election which saw the return of Milton Obote to power, and which is widely considered to have been rigged.
Dr Besigye told the East African newspaper that he did not immediately join Mr Museveni in the bush war.
But he was imprisoned for two months in the Nile Hotel in 1981, accused of working with the rebels, and tortured.Death threats
End Quote Kizza Besigye
I left in order to continue to be politically active rather than being behind bars or six feet under as had been threatened”
Dr Besigye fled to Nairobi and in 1982 joined Mr Museveni in the bush, where he became his personal doctor.
Although rarely at the battle front, Dr Besigye was deployed in divisions which were sent into battle where he treated the casualties.
When Mr Museveni came to power, Dr Besigye, aged just 29, was appointed state minister of internal affairs and national political commissar.
These appointments shocked some who had been in the bush with Dr Besigye, as he had not been heavily involved in the political side of the bush war.
Some believe Dr Besigye's rapid rise led to him being viewed as a potential threat to the president, and as a result he was given less demanding work during the 1990s.
Dr Besigye rose to the rank of colonel in the army but did not retire from it until shortly before the 2001 elections - having written a document that accused the ruling National Resistance Movement of being undemocratic, dishonest and corrupt. It almost earned him a court martial.
A few months before the election, he emerged as a presidential candidate.
Having lost to Mr Museveni, he went to court and challenged the result on the grounds that the state had used force, intimidation and violence.
He lost the case on a split decision and then fled the country.
On his return in 2005 he said: "I left in order to continue to be politically active rather than being behind bars or six feet under as had been threatened."
After he returned to Uganda ahead of the 2006 elections, he was arrested and charged with both treason and rape.
The charges were later dismissed, but not until long after the polls and they obviously distracted him from campaigning.
The rape prosecution was dismissed as "crude and amateurish" but the treason charges hung over his head until 2010.
His lawyers unsuccessfully challenged the 2006 results, alleging widespread fraud. Dr Besigye won 37%.
The government says Dr Besigye has connections with a rebel group, and this accusation led to charges.
He denied any rebel link, although he has said he would not rule out going to the bush to battle the government if the constitution was overthrown and needed to be reinstated.
His critics see him as power hungry. But others praise him for standing up to authority.
Dr Besigye himself says his mission is "to work with millions of other Ugandans in bringing about a stable democratic and peaceful Uganda".