Police detained Uganda's main opposition candidate Kizza Besigye for the third time this week and clashed with his supporters as early contested election results showed President Yoweri Museveni set to extend his 30-year grip on power.
On Friday (local time) officers set off stun grenades and fired tear gas at crowds outside the headquarters of Besigye's Forum for Democratic Change, where senior members of the party accused organisers of rigging the vote.
Police later bundled Museveni's chief challenger into a van as the party was preparing to hold a press conference, a Reuters witness said.
Officers said he was about to announce unofficial preliminary results - one commander said he had not been arrested but simply taken home.
Museveni, one of Africa's longest-serving leaders, has presided over strong economic growth, but faced mounting accusations at home and abroad of cracking down on dissent and failing to tackle corruption.
Other clashes broke out across the capital, and the spokesperson for another presidential contender, former prime minister Amama Mbabazi, said police had barricaded him in his home, stoking tensions a day after the vote.
That report could not be confirmed independently.
Early results showed Museveni, a Western ally against security threats including Islamist militants, with a sizeable lead of 62 percent based on a count of ballots in 42 percent of polling stations.
Besigye trailed with 33 percent of the vote, and Mbabazi with 2 percent.
But Dan Mugarura, a senior official from Besigye's party, said there were "glaring discrepancies" compared to reports from polling stations.
Besigye, who challenged Museveni unsuccessfully in three previous elections, has repeatedly said the election would not be free and fair.
The Electoral Commission has regularly denied accusations of anti-opposition bias.
Late on Thursday, Besigye was briefly detained in Kampala for alleged criminal trespass and assault.
US State Department spokesman John Kirby said it called into question Uganda's commitment to a transparent election, free from intimidation.
The 71-year-old sitting president had earlier warned that anyone caught stoking violence would face the wrath of Ugandan security forces, who were deployed in heavy numbers across capital Kampala in riot gear.
Election day was otherwise largely peaceful, although voting was delayed in some areas, especially in the capital.
The government, citing security fears, said it shuttered the social media such as Facebook and Twitter, though all were working again on Friday.