Uganda's incumbent President Yoweri Museveni has won a presidential election in the country despite criticism of the process by European and Commonwealth observers.
President Museveni received 60.7 percent of the vote while his closest rival Kizza Besigye gathered 35.4 percent, the electoral commission announced.
Ugandan police arrested Kizza Besigye on Friday as preliminary results from the presidential election showed Museveni set to extend his 30-year grip on power.
He appeared to be under house arrest on Saturday.
"The results of the presidential elections must be rejected," he said in a statement.
"We have just witnessed what must be the most fraudulent electoral process in Uganda."
Electoral commission chairman Badru Kiggundu stated that there may have been "minor errors" in the electoral process.
"We are a very young democracy but we are improving to international standards," he said.
Eduard Kukan, chief observer for the EU Mission, told journalists in Kampala that the result was damaged due to a "lack of transparency and independence" in the Ugandan electoral commission.
"State actors created an intimidating atmosphere for both voters and candidates," he added.
Museveni, 71, took power by force in 1986 and pulled Uganda out of years of chaos. He is a key US ally on security matters, especially in Somalia. But his critics worry he may want to rule for life and accuse him of using the security forces to intimidate and harass the opposition.
Besigye, 59, was Museveni's personal physician during a bush war and served as deputy interior minister in Museveni's first Cabinet. He broke with the president in 1999, saying Museveni was no longer a democrat.
The opposition had tried to appeal to angry young voters, particularly in urban places, where rate of unemployment is high and many are discouraged by the poor state of schools and hospitals.
"We are disappointed, the election has been rigged," said 23-year-old Brenda in Kampala, who refused to share her second name due to fearing for her security.
"I have never seen another president and it seems it will be like that until he dies."