The Opposition has called the election results fraudulent.
A European Union mission urged Uganda on Thursday to release detailed results from last week's presidential election, which extended President Yoweri Museveni's 30-year rule but which the opposition has called fraudulent.
Uganda's Electoral Commission declared Museveni, 71 and in power since 1986, the winner of the February 18 vote with about 60% of the vote.
The EU's Election Observation Mission statement was released soon after an aide to Kizza Besigye, the main challenger, said Besigye had been arrested for the sixth time in about a week.
Besigye, who challenged Museveni in three previous elections, was also blocked from leaving his house on Wednesday, when local elections were held across Uganda. Human rights groups say they have been blocked from meeting with him.
The EU mission "encourages the Electoral Commission to publish without delay the detailed results from each polling station" so that voters and the candidates can "comprehensively evaluate the election results," it said in a statement.
"The electoral process is completed only when all candidates have rightfully exercised the relevant legal avenues without fear, intimidation or other undue restrictions," Eduard Kukan, the chief observer, said in a statement.
Candidates have 10 days after a winner is declared to contest an election's outcome. A spokeswoman for Besigye said his party has yet to decide whether it will formally dispute the results, but said Besigye's detentions are creating an obstacle to its activities.
"The arrest of our candidate ... has eaten some of the time we have to lodge a petition," said Ingrid Turinawe. A second party official said that police had arrested 21 party officials since Tuesday in various of the country.
Police prevented Besigye from travelling to vote in local council elections because they suspected he wanted to "link up with his supporters to engage in illegal protests", police spokesman Patrick Onyango said.
The other party officials were arrested because they were mobilising people to travel to Kampala and "cause violence in the city", Onyango told Reuters. Critics say Museveni, who has been lauded for restoring economic and political stability to Uganda after years of turmoil, hopes to stay in power for life.
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