(CNN) — Tensions mounted as votes in Uganda are counted in an election that saw the detention of the leading opposition candidate, a ban on social media and allegations of voter fraud.

Ugandans voted on Thursday on the fate of President Yoweri Museveni, who is hoping for a fifth term after ruling the nation for the last 30 years.

The day began with many politically-minded voters unable to tweet or update their Facebook pages after a government ban on social media.

After casting his vote at his home in western Uganda, Museveni defended the ban as a “security measure to avert lies … intended to incite violence and illegal declaration of election results.”

Human rights groups condemned the social media clampdown. Many citizens accessed social media sites via encrypted private networks. A similar shutdown occurred during the 2011 general election.

“The Ugandan government’s decision to block access to social media on mobile phones on election day is a blatant violation of Ugandans’ fundamental rights to freedom of expression and to seek and receive information,” said Sarah Jackson, Amnesty International’s deputy regional director. “Without clearly defined security concerns, this closure is nothing but an exercise in censorship as Ugandans elect their leaders.”

Opposition candidate briefly held

Throughout the election, social media has been used as a rallying and debate tool.

In January, Ugandans used the hashtag #1986pictures with tweets of 30-year-old pictures and remarks like “In 30 years, everything has changed in Uganda except the president.”

Further complicating the vote, opposition candidate Kizza Besigye was detained in the area of Kasangati near the capital, Kampala, on Thursday, local police told CNN.

Besigye, who was released after about three hours, was detained after attempting to enter a call center he suspected was being used to store illegal election materials, Kampala police spokesman Patrick Onyango said.

Accompanied by journalists and members of a European Union observer mission, Besigye arrived at the secure police facility and demanded to be let in, Onyango said. Besigye has been arrested on numerous occasions in his bid for the presidency.

The mayor of Kampala, Erias Lukwago, was arrested in his home in the area of Rubaga, police said. The reason for the arrest is unclear.

The start of the election was delayed in in some polling stations because of what the electoral commission said was a shortage of election materials. The Commonwealth observer mission’s leader, former Nigerian President Olusegun Obasanjo, called the delays “inexcusable.”

Polls closed at 6 a.m. ET, with results expected Friday.

Uganda’s Electoral Commission reported that a candidate of the ruling National Resistance Movement was arrested for ballot stuffing in the Busiro South constituency south of Kampala. The ballots
stuffed in various boxes had votes for him and Museveni, the commission reported.

In the lead-up to the election, rights groups have warned that freedom of association and expression have been under threat.

“Uganda has seen increased levels of authoritarianism in the last decade,” said Magnus Taylor, Horn of Africa analyst at International Crisis Group. “This has come in waves, most evident during election period when a generally strident opposition has clashed against the president’s anti-democratic tendencies.”

Few expect new leader to emerge

A crowd at one polling station in Kampala surged at a man they say had marked ballots in favor of Museveni. Police and military used tear gas and live bullets to disperse the crowd.

Onyango said nine people were injured and 20 arrested.

Now 71, Museveni has been in power since 1986. In 2005, the constitution was changed to allow him to extend presidential term limits.

Few observers expect Museveni to lose. Taylor of the International Crisis Group said a victory signals an ominous trend.

“A re-election for Museveni would signal the persistent advantages incumbents have in controlling the political process, making it very difficult for opposition parties or candidates to compete with national structures, finance and support from partisan government institutions,” he said.

“It would be … ‘business as usual’ for Uganda and Africa in general, where the value of incumbency is frequently in evidence.”

Leaders across Africa have been criticized for extending their term limits to stay in power. Rwanda amended its constitution to allow the leader to potentially rule until 2034. In Burundi, President
Pierre Nkurunziza’s June decision to amend the constitution in order to run for a third term ignited unrest, resulting in more than 300 deaths.

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