JINJA, Uganda — Police arrested Uganda’s leading opposition presidential candidate Thursday, just as results started to trickle in from an election that focused on whether the country’s ruler would extend his 30-year grip on power.
The brief detention of opposition leader Kizza Besigye came at the end of an election day marred by irregularities, including ballots delivered hours late and an apparent shutdown of social media. Besigye was arrested on charges of criminal trespass and assault and was shortly thereafter released on bond, authorities said.
The president, Yoweri Museveni, 71, is one of the world’s longest-ruling leaders, a former rebel who has become a key U.S. ally in East Africa, even as security forces under his control have jailed members of the political opposition.
In Washington, State Department spokesman John Kirby condemned Besigye’s detention. “Such an action calls into question Uganda’s commitment to a transparent and free election process, free from intimidation,” he said.
Besigye was once Museveni’s personal physician and served in one of the president’s early cabinets. He was running against Museveni for the fourth time.
In the previous three races, Besigye never polled higher than 38 percent, though he has disputed the returns. At a news conference this week, he called the election a “struggle for democracy.”
Uganda receives $750 million annually in aid from the United States, including significant investments in military training and support. The country also has played a key role in regional, U.S.-supported peacekeeping missions, at Museveni’s behest.
He has sent thousands of troops to the ongoing African Union mission in Somalia that is confronting al-Shabab, an Islamist militant group linked to al-Qaeda.
Opinion polls before the election showed Museveni with a comfortable lead, although opposition figures have questioned the polls’ reliability. For its part, the country’s Electoral Commission promised a free and fair election.
Besigye’s arrest came outside a home in the capital, Kampala, where he had gone to expose what he alleged was a vote-rigging operation, according to his party spokesman, Ibrahim Nganda.
According to officials, the house was actually a police security facility and, in the course of attempting to enter the house, Besigye assaulted a police officer, harming him.
Nganda dismissed the accusation, saying that “they always concoct charges” when political figures are arrested.
Besigye was also detained briefly in Kampala on Monday after his convoy attempted to pass through the city center. Police said the candidate was arrested because his route posed a security risk and had not been negotiated ahead of time. At least one person was killed in the running protests that followed, and security forces remained deployed across the city.
In Thursday’s election, voters were also electing representatives to the 385-member parliament. The legislature is now controlled by Museveni’s National Resistance Movement party.
Voting stations opened late — in some cases many hours late — and social-media sites appeared to be blocked on some networks.
In some areas of Kampala, anger over delays spilled into the streets. In one neighborhood, voters who said they had waited seven hours for the arrival of ballots held a demonstration, marching with an opposition banner toward a polling station. Security forces fired tear gas to disperse the protesters. At least two polling stations in the area were shut down.
In Jinja, a small town near the Nile River’s source on Lake Victoria, voters turned out early Thursday. An election monitoring group had warned that western Jinja was one of the areas most likely to have violence around the voting.
There was frustration as some stations remained closed more than four hours after the official 7 a.m. start. Election workers said they were still waiting for materials to arrive from regional Electoral Commission officials.
Ibrahim Ayakiza was the first in line at the Mpumudde High School polling station on the outskirts of town. He said the late start raised concerns about political maneuvering.
“I think the delay is so some of us quit and go away,” he said.
Adding to the unease in the country, Twitter and Facebook appeared to be blocked on some Internet providers. The networks have been flooded with rumors about attempts to rig the vote, including conspiracy theories that ballot-station pens would be filled with ink that would disappear before votes could be counted.
“A lot of negativity, lies, are being beamed down to create unnecessary tension,” the police inspector general, Kale Kayihura, said of social media.
One of the providers, the telecommunications giant MTN, called the disruption a “temporary interference” on Twitter.
The Electoral Commission pledged that all voters would have the opportunity to cast a ballot. Polling stations were scheduled to close by 4 p.m., but authorities extended voting in the capital and nearby areas for three hours. The commission will have 48 hours to count the votes and announce a winner.
Former Nigerian president Olusegun Obasanjo, who is heading the Commonwealth Observer Group monitoring the vote, said in a statement: “A delay of an hour or two is excusable. Delays of three, four, five and even six hours, especially in Kampala, are absolutely inexcusable and will not inspire trust and confidence in the system and the process.”
In addition to Besigye, Museveni also faced a challenge from former prime minister Amama Mbabazi, who split with the ruling party in 2014.
Kevin Sieff in Nairobi contributed to this report.