Besigye's FDC party has described the crackdown on the opposition as a "creeping military coup." Uganda's president claimed "Europeans are not serious" about democracy amid increased calls for transparency and dialogue.
Ugandan police on Monday arrested opposition leader Kizza Besigye for the fourth time in eight days after losing to long-serving President Yoweri Museveni in Thursday's elections.
Besigye was placed under house arrest on Friday, with authorities warning him of adverse consequences if he chose to lead a planned protest march through the capital Kampala on Monday.
"Today, [Besigye] had mobilized a group of youth to storm the electoral commission. We had information that they had planned to cause violence in city," said Ugandan police spokesman Patrick Onyango.
The opposition leader said there were widespread voting irregularities in Thursday's elections, a claim supported by international election monitors.
Besigye's Forum for Democratic Change Party (FDC) lashed out at his latest arrest, describing Museveni's rule as a dictatorship.
"We are witnessing a creeping military coup that aims to subvert the will of the people," the FDC said in tweet.
"The most brutal dictatorships had crumbled when the people asserted themselves. Museveni's is no exception, stand up for your rights," the party added.
Uganda's Electoral Commission declared Museveni, who has been in power since 1986, the winner of Thursday's election, adding that Besigye only managed to garner 35.4 percent of the vote.
'Europeans are not serious'
Meanwhile, the EU delegation to Uganda noted that there were "shortcomings in the election process, notably in the areas of neutrality, transparency and the effectiveness of the election administration amongst others."
"Taking into police custody of opposition leader Mr. Kizza Besigye several times is contrary to basic democratic principles," a statement from the EU's observer mission said.
Museveni criticized the EU delegation's statement on voting irregularities and described the opposition as "liars."
"The opposition are not leaders. They are just demagogues, liars, just talking, talking," the president said in a statement on Sunday.
"I told those Europeans … I don't need lectures from anybody," Museveni said, adding: "Those Europeans are not serious."
His statement comes as US Secretary of State John Kerry condemned the harassment of opposition figures.
ls/jil (AFP, Reuters, dpa)