Incumbent, Yoweri Museveni has been declared the winner of the Uganda presidential election, with 58.64% of the total votes, the country's electoral commission announced.
His main opposition, Bobi Wine, has alleged vote-rigging throughout the process.
The singer-turned-politician told the BBC that he rejected the results "with the contempt they deserve".
Wine alleges there had been a lot of irregularities but Mr Museveni called it Uganda's fairest ever vote.
Mathias Mpuuga, a member of Wine’s National Unity Platform, told a news conference: “We have evidence of ballot stuffing and other forms of election malpractice and after putting it together we are going to take all measures that the law permits to challenge this fraud.”
In a statement Africa Elections Watch, a coalition of civil society groups, which said it had 3,000 election observers in Uganda, said that the vote did not "meet the threshold of a democratic, free, fair, transparent and credible electoral process".
Museveni, 76, who has ruled Uganda since 1986, when he helped to end years of tyranny under Idi Amin and Milton Obote, claimed a sixth five-year term, extending his rule to four decades.
The clashes with police following the announcement of Museveni’s victory took place in a number of locations, including in Gomba, where Wine lives, and neighbouring Sembabule. They were quickly dispersed.
Meanwhile, Mr Wine, whose real name is Kyagulanyi Ssentamu, says his life is being threatened.
He tweeted on Sunday that military units are not allowing him and his wife, Barbie, from leaving their house, not even to harvest food from their garden.
“It’s now four days since the military surrounded our home and placed my wife and I under house arrest,” said Wine’s tweet.
“We have run out of food supplies and when my wife tried to pick food from the garden yesterday, she was blocked and assaulted by the soldiers staged in our compound.”.
Addressing what his party, National Unity Platform, might do now, Wine said that, "all options" are now on the table including peaceful protests but stressed that he was not calling for violent insurrection.
Although Mr Museveni holds on to power, at least 15 of his cabinet ministers including the vice president Edward Ssekandi were voted out, with many losing to candidates from Mr Wine's party, according to local media.