Uganda's President Yoweri Museveni (R) casts his ballot as he votes in parliamentary and presidential elections at a polling station in Nshwere, on February 18, 2016
Kampala (AFP) - Partial results in Uganda's chaotic election on Friday gave veteran president Yoweri Museveni a solid lead as the US urged him to "rein in" the police after his main challenger was detained for the third time this week.
As voting concluded on Friday, provisional results gave the 71-year-old leader 63 percent of the vote with nearly half of polling centres counted.
His chief rival Kizza Besigye -- in second place with 33 percent -- was arrested again on Friday after police stormed his party headquarters in the capital.
The police raid came as people continued to vote at dozens of polling stations in the wider Kampala area, where voting was extended in 36 sites for a second day after delays on Thursday that Commonwealth election observers called "inexcusable".
While Museveni maintained his lead in the presidential tally, at least 17 of his ministers lost their parliamentary seats, among them defence minister Crispus Kiyonga -- who is spearheading regional efforts to end the political crisis in Burundi -- and attorney general Fred Ruhindi.
The former rebel fighter faced a challenge from seven candidates, but is expected to win re-election for a fifth term in office and extend his 30-year rule of the east African country.
- Opposition HQ raided -
Thursday's presidential and parliamentary votes were disrupted in the capital Kampala by the late arrival of ballot boxes and papers, angry demonstrations by frustrated voters and police using tear gas.
At nearly 28,000 other polling centres voting passed off smoothly on election day.
Besigye, who was arrested during campaigning on Monday and again on Thursday evening, was taken into custody for a third time on Friday.
Police surrounded Besigye's Forum for Democratic Change (FDC) headquarters, firing tear gas and water cannon, before bursting inside and arresting top party officials.
"Some reasonable measures have been applied to rein in FDC supporters who wanted to disturb the peace and the ongoing exercise," said senior police officer Felix Andrew Kaweesi.
He accused Besigye's FDC of planning to publish its own tally of results, contravening electoral law.
US Secretary of State John Kerry, visiting London, voiced concern.
"(Kerry) urged President Museveni to rein in the police and security forces, noting that such action calls into question Uganda's commitment to a transparent and credible election process free from intimidation," the State Department said in a statement.
Kerry also urged Museveni to remove blocks on popular social media and mobile money sites that had been put into place on Election Day.
- 'Serious shortcomings' -
The Citizens Election Observers Network-Uganda (CEON-U) said the election had been "relatively peaceful" but noted "some serious shortcomings in adherence to (international) standards".
Kerry said he was encouraged that hours at some polling sites had been extended.
Over 15 million Ugandans were registered to vote for both a president and members of parliament, with 290 seats contested by candidates from 29 political parties.
The leading presidential candidate requires more than 50 percent of votes cast to avoid a run-off with initial full results expected as early as Saturday.
Disenchantment with politicians, their politics and the faltering economy has been increasing in a country, where the median age is less than 16.
Voter turnout has followed a downward trajectory in recent elections with nearly three-quarters of eligible voters casting a ballot in 1996, during the country's first-ever competitive election, but only three-fifths turning out in 2011.
Museveni's share of those votes has also declined although he won his last five-year term in 2011 with 68 percent.