KAMPALA, Jan. 15 (Xinhua) -- Uganda's general elections kicked off on Thursday with more than 21 million voters casting ballots for the presidency and parliament amid tight security.

According to the Electoral Commission, the country's electoral body, voting was scheduled to start at 7:00 a.m. local time (0400 GMT) and close at 4:00 p.m. local time (1300 GMT).

More than 21 million Ugandans are expected to cast their ballots to elect the president and 353 directly elected members of parliament.

Elections for members of parliament representing special interest groups, including older persons, persons with disabilities, workers, and youths, will be held at a later date.

In the capital, Kampala, and some other parts of the country, voting was delayed due to network failures affecting biometric voter verification machines.

Speaking to state-owned broadcaster Uganda Broadcasting Corporation Television, Electoral Commission Chairperson Simon Byabakama said polling officials had been instructed to rely on manual registers following the technical failures.

Eight candidates are vying for the presidency, including incumbent President Yoweri Museveni, who is seeking a seventh five-year term of office.

"I thank Ugandans for turning up and casting their votes. The voter verification machines had problems at first, but they are now okay," Museveni said soon after voting in his home district, Kiruhura, western Uganda.

Museveni's main challenger, pop star-turned-politician Robert Kyagulanyi, cast his vote in Magere in Wakiso District. He told reporters that despite the arrest of several supporters, he remained confident of victory and urged voters to turn out in large numbers.

"I call upon all of you who have not voted to rush to the polling stations and vote. Your vote is your voice, and you must not allow anybody to suffocate it," Kyagulanyi said.

According to the Electoral Commission, a presidential candidate must secure more than 50 percent of the vote to win outright. Results are expected within 48 hours after polls close.

Xinhua reporters observed that security remained tight but calm in Kampala, with armed personnel deployed on foot and motorized patrols.

While the United Nations Human Rights Office has criticized the election process, citing arrests of opposition supporters, the Ugandan government has dismissed the allegations.

Election observers from the Forum of Parliaments of the Member States of the International Conference on the Great Lakes Region said the vote is critical to Uganda's political stability and multiparty democracy, noting the country's central role in regional peace and security.