The vote will come after one of the bloodiest campaigns in years, as veteran leader Yoweri Museveni seeks a sixth term against the 38-year-old popstar-turned-MP Bobi Wine, who has managed to fire up a youthful population that has mostly known only one president.
Wine, who has spent most of the campaign in a bulletproof vest and combat helmet facing teargas, bullets and numerous arrests, on Tuesday urged his supporters to "protect" the election from rigging.
Despite Wine's popularity, few observers believe he can pull off a victory against Museveni, a 76-year-old former rebel leader who has ruled since 1986 and effectively crushed any opposition.
Facebook, WhatsApp and Twitter were all down in the East African nation on Tuesday, after the communications social media shutdown order.
'Nasty and aggressive'
In a letter seen by AFP, Uganda Communications Commission executive director Irene Sewankambo ordered telecommunications companies to "immediately suspend any access and use" of social media and online messaging platforms.
An industry insider who spoke to AFP on condition of anonymity said that the order was first communicated in "nasty and aggressive" phone calls to the telecommunications companies on Tuesday morning.
The calls made it clear the order was in retaliation after Facebook deleted pro-government accounts for seeking to manipulate public debate ahead of the election.
Facebook said on Monday that the accounts were linked to the ministry of information and technology.
However UCC spokesman Ibrahim Bbosa told AFP: "I am not aware of a directive to switch off internet or social media platforms."
He blamed any problems on heavy use of internet ahead of the election saying "There has been slow connectivity on the platforms which can be partly due to heavy traffic as a result of the forthcoming elections ».
Wine says staff beaten
Some 18 million voters are registered for the presidential and parliamentary ballot, which takes place after a chaotic campaign
European Union High Representative Josep Borrell said in a statement calling for a credible vote that "the excessive use of force by law enforcement and security agencies has seriously tarnished this electoral process."
Wine, whose real name is Robert Kyagulanyi, told journalists his home had been raided and his staff beaten by security forces on Tuesday morning.
He 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 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."
Museveni faces a divided opposition in this election, with 10 candidates battling him.
(with wires)
