Uganda's opposition frontrunner on Tuesday urged supporters to turn out in large numbers and to "protect" this week's election from rigging, accusing the ruling party of trying to intimidate voters.

The appeal from Bobi Wine came as the popstar-turned-presidential aspirant alleged his home was raided and staff beaten by security forces ahead of Thursday's election.

Wine, 38, has emerged the main challenger to 76-year-old President Yoweri Museveni, who has ruled since 1986 and is seeking re-election after constitutional changes allowed him to seek a sixth consecutive term.

The veteran leader has been accused of clinging to power and skewing the playing field ahead of the vote.

Contenders: President Yoweri Museveni, left, and musician-turned-politician Robert Kyagulanyi, also known as Bobi Wine
Contenders: President Yoweri Museveni, left, and musician-turned-politician Robert Kyagulanyi, also known as Bobi Wine AFP / Sumy Sadurni

Campaigns have been suspended in opposition strongholds, and Museveni's rivals denied permission to rally, under the pretext of containing the coronavirus.

The electoral commission, again citing Covid-19 regulations, has ordered the 18 million Ugandans registered in the presidential and parliamentary ballot to leave polling stations quickly after casting their vote.

Wine, whose real name is Robert Kyagulanyi, said the ruling party was trying to scare voters away from the ballot box and urged them to record any abuses or irregularities on polling day.

"We encourage all voters to stay and guard the vote," he told reporters in Kampala, flanked by two other opposition candidates.

"We are telling you, you will not be breaking the law when you stay and protect your vote. We encourage you to use your phones, use your cameras. Your phone is a very powerful weapon, that camera is very powerful, use it."

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People walk in a parade as they celebrate the annulment of an anti-homosexuality law by Uganda's constitutional court in Entebbe August 9, 2014. reuters/Edward Echwalu

The run-up to the vote has been marred by some of the worst pre-election violence in years with dozens killed in street protests in November.

Wine said that his home was raided Tuesday morning and staff beaten and detained, just days after he sent his family to the United States, citing safety concerns.

The popular musician, who turned to politics in 2017, has been arrested and harassed by police throughout the campaign, and donned a bulletproof vest and combat helmet after complaining of being repeatedly shot at.

Four-time presidential candidate Kizza Besigye, himself a veteran target of campaign violence, described the brutality directed at opposition candidates this election as "unprecedented".

"It seems to be scaled up every coming election," said Besigye, who is not running this year.

"That is I believe not surprising, because the lower the support for the regime goes, the more they replace it with repression and violence."

Access to Facebook, WhatsApp and Twitter was disrupted in Uganda on Tuesday.

Last week, Wine's party launched an app to track election results in tandem to government tallies but access to the service has been disrupted.

Facebook announced on Monday it had shut a slew of accounts belonging to Ugandan government officials accused of seeking to manipulate public debate ahead of the elections.