Dr Besigye’s sight was damaged by pepper spray when he was arrested two weeks ago Ugandan opposition leader Kizza Besigye says he has been prevented from returning home from Kenya after receiving medical treatment in Nairobi.
He told the BBC the Ugandan government said the flight would not be allowed to land with him on board.
A Ugandan government minister has denied such an order was given.
Dr Besigye was President Yoweri Museveni’s main opponent in February’s election. Mr Museveni is to be sworn in on Thursday for a fourth term.
Dr Besigye was violently arrested during a protest in Uganda two weeks ago and his eyesight was damaged by pepper spray.
He told the BBC from Nairobi’s Jomo Kenyatta international airport:
“The Ugandan government had told the Kenya Airways chief executive that if they board us the aircraft will not be allowed to land in Uganda and that we were not welcome and that therefore they had absolutely nothing to do if they wanted to fly to Uganda but offload us.”
Ugandan Internal Affairs Minister Kirunda Kivejinja said the government had no authority over who boarded Kenya Airways flights.
With President Yoweri Museveni’s swearing in due on Thursday, it seems the government would rather keep Dr Kizza Besigye away.
The fear is that opposition supporters might choose to show that they have had enough of 25 years of President Museveni and take to the streets, which would be a severe embarrassment for the president in front of visiting heads of state.
With Kizza Besigye out of the country the likelihood of protests diminishes considerably.
The question now is how Dr Besigye’s supporters will react to what the opposition is describing as his enforced exile.
Delaying his return may lead to a larger show of support when he is allowed back – a risk the government seems willing to take in order to ensure there is no disruption to the swearing-in ceremony, which is set to cost over $1m.
On the streets of Entebbe and Kampala the well-armed police force is highly visible – a message of military might intended to keep the opposition out of sight.
“If we managed to allow him to leave, how can we stop him from coming back?” he told Uganda’s Daily Monitor.
It had earlier appeared as though the police and the opposition had reached an agreement on Dr Besigye’s return home, says our correspondent.
But the return of Dr Besigye in time for Mr Museveni’s inauguration had presented an awkward scenario, our correspondent adds, with the possibility of his supporters gathering in large numbers.
The security forces have been deployed in large numbers in Entebbe and along the 40km (25 miles) route into Kampala.
In the last week, the opposition has stepped up its campaign over high prices, which has led to recent clashes between the police and protesters.
Kizza Besigye’s car was attacked during a demonstration in Kampala on 28 April. He was doused in pepper spray and suffered injuries to his eyes.
On Tuesday, opposition demonstrators were doused in pink liquid by police as they tried to hold a banned rally in Kampala.
President Museveni has said that, in an effort to curb riots, he wants a new law to deny bail for six months to those arrested while protesting.
Mr Museveni has been in power for 25 years and was re-elected to a fourth term in February in polls that Dr Besigye says were rigged.
The two used to be colleagues, with Dr Besigye serving as Mr Museveni’s personal physician when they were in the bush fighting President Milton Obote’s government in the early 1980s.
Dr Besigye had a role in government when Mr Museveni took power but they fell out about 10 years ago.
Dr Besigye left the country but returned in 2005 with the introduction of multi-party politics.
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