International election observers warned Thursday that hours-long delays in delivering ballot papers in Uganda's national elections would not "inspire trust" in the polling.

"A delay of an hour or two is excusable. Delays of three, four, five and even six hours, especially in Kampala, are absolutely inexcusable and will not inspire trust and confidence in the system and the process," Olusegun Obasanjo, the head of the Commonwealth Observer Group in Uganda, told AFP.

Obasanjo, a former Nigerian president, is leading a 13-member team from across the Commonwealth.

Voting in Uganda was due to begin at 07:00 am (0400 GMT) but was stalled for several hours in some polling stations in parts of the city and the surrounding Wakiso district, where ballot boxes and papers did not arrive on time.

Support for the opposition is traditionally strong in the capital. Some frustrated voters accused the authorities of deliberately stalling the vote, and police fired tear gas in the capital to disperse angry voters.

As ballot counting began, others were still queuing to vote in the capital.

The Electoral Commission said all those standing in line when polls were due to close at 4:00 pm (1300 GMT) would be permitted to vote, and that polling centres where materials were delivered late in Kampala and Wakiso would stay open until 7:00 pm (1600 GMT).

Social media, including Facebook and Twitter, were largely inaccessible on voting day although Internet-savvy Ugandans dodged the apparent shutdown using virtual private networks.

Government regulator the Uganda Communications Commission said the attempted shutdown was for "security reasons", without giving details.

Obasanjo said any shutdown was "ill-advised."