Uganda's electoral commission has apologised over delays in opening polling stations as people queue to vote in crucial presidential elections.

Difficulties in transporting electoral materials caused the delays, it said.

President Yoweri Museveni, 71, is seeking to extend his 30-year rule, in a race widely seen as the tightest in the East African state's history.

Seven opposition candidates are running against him. His main challenger is his former physician, Kizza Besigye.

Mr Besigye has said he does not think the election will be free and fair.

Former Prime Minister Amama Mbabazi is also seen as a leading contender for the presidency.

Almost five hours after the vote was meant to begin, the electoral commission issued a statement acknowledging that some polling stations had not yet opened.

It added that despite the delays, voting will not be extended.

The BBC's Catherine Byaruhanga reports from the capital, Kampala, that thousands of people who had queued for hours got visibly frustrated over the delays.

Security forces were sent in to calm the situation, she adds.

A candidate needs to secure more than 50% of the vote to win outright and avoid a run-off with the second-ranked contender.

Ugandans are also voting in parliamentary and local elections.

Human rights groups have reported an intensifying government crackdown on opposition leaders, their supporters and the media during the election campaign.

One person was killed on Monday in clashes between Mr Besigye's supporters and the security forces in the capital, Kampala.

Mr Museveni has been in office since winning a five-year guerrilla war in 1986, and he is one of Africa's longest-serving leaders.

His final term was meant to end in 2006, but in 2005 he won a campaign to lift the constitutional term limits.

Mr Besigye has unsuccessfully challenged his former comrade-in-arms in the last three presidential elections

Mr Mbabazi is another former ally of President Museveni. He served as prime minister in 2011-2014, when he was dismissed after announcing he would launch a rival presidential bid.

During the election campaign the two opposition candidates accused the president of corruption and criticised his economic policies.

Mr Museveni dismissed the allegations as fiction.

Major issues for voters also include persistently high unemployment and poor quality of public services.