A 38-year-old pop star is challenging one of Africa’s longest serving leaders in Uganda’s hotly contested election.

Robert Kyagulanyi, known by his stage name Bobi Wine, says he represents the country’s younger generation, while Yoweri Museveni, 76, says he is standing for stability.

The campaign has been marred by serious violence, which has seen dozens of people killed.

The government has ordered a block on all social media.

President Museveni says this was because Facebook had banned several accounts which backed his ruling party.

Ugandan police say they will deploy officers on rooftops of the capital Kampala on election day, while armoured vehicles have started patrolling the streets.

Polls open at 07:00 local time (04:00 GMT), with results not expected before Saturday.

President Museveni is seeking his sixth elected term in office after 35 years in power.

Opposition candidates – including Bobi Wine – their supporters and campaign staff have been arrested on several occasions.

The police say the deployment to shopping malls and other high-rise buildings comes after opposition activists commanded protests from rooftops in November, when more than 50 people were killed after Bobi Wine was arrested.

The US announced on Wednesday that it was withdrawing its election observers after most of its accreditation requests were denied.

The US ambassador to Uganda Natalie Brown said in a statement that without observers, the presidential and parliamentary elections would lack “accountability, transparency and confidence”.

She added that numerous Ugandans had also not received their observer accreditation yet.

The European Union’s offer to send electoral experts has not been taken up either.

In response to this criticism, Mr Museveni’s spokesman Don Wanyama tweeted that there were observers from the African Union and East African Community.

“I don’t remember when Uganda last sent election observers to the US,” he added.

The government has previously said the election would be free and fair.

Bobi Wine has called on voters to remain at polling stations on Thursday and use their mobile phone cameras to record the tallying process in an effort to prevent vote rigging.

On Wednesday, telecoms companies told their customers that they had blocked all social media and messaging apps indefinitely, following an order from Uganda’s Communications Commission.