Ugandan opposition groups have called for a boycott of Thursday's referendum on restoring multi-party democracy, after their legal action failed.
Critics accuse Yoweri Museveni of seeking to be "president for life"
The G6 opposition coalition say the vote is costly and illegal.
The Supreme Court ruled it could not deal with the issue. A yes vote would end a 19-year ban on political parties ahead of elections next year.
Critics say the referendum is a smokescreen for keeping President Yoweri Museveni in power.
Last month MPs backed a controversial amendment to the constitution, which scrapped limits on presidential terms.
Since Mr Museveni came to power 19 years ago, Uganda has operated a unique political system which severely restricts political parties.
Five years ago, Ugandans rejected a return to multi-party politics when Mr Museveni campaigned against it.
This time he is campaigning in favour.
He introduced the "Movement" system to prevent sectarian violence.
The G6 comprises the Democratic Party, the Forum for Democratic Change, the Uganda People's Congress, the Justice Forum Party, the Conservative Party and the Free Movement.
They say their rights to operate freely should not be put to a vote, but have been illegally denied by the president.
Some donors have cut aid to Uganda in protest at the slow pace and direction of democratic reforms.
Police have stepped up security ahead of Thursday's vote which takes place between 0400 GMT and 1400 GMT.