The head of Uganda's media regulator, Godfrey Mutabazi said the block was introduced after a request from the electoral commission, However, many users have found a way around the controversial restrictions as #UgandaDecides is still trending on Twitter.
Uganda's leading mobile operator MTN, which has 10 million users, confirmed that it had received an order to shut down social media services, which include WhatsApp, Facebook and Twitter.
Mobile money services has also been shut to ensure they are not used to bribe voters.
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.
His main challenger is his former physician, Kizza Besigye who 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.
The election has been marred by delays of up to five hours at some polling stations.
The electoral commission said difficulties in transporting electoral materials caused the delays.
It added that despite the delays, voting will not be extended.
There have been riots in some parts of Uganda's capital,Kampala, following the arrest of opposition leader Kizza Besigye. The army and police raided the headquarters of his FDC party just before Mr Besigye was to address a press conference.
The US embassy in Uganda has criticised the police following the raid on the headquarters of the opposition FDC and also warned US citizens in Uganda to ''stay off the streets''.