Ugandan MPs have voted overwhelmingly in favour of a constitutional amendment allowing President Yoweri Museveni to seek further terms in office.
Ugandans fear Museveni stay on for life
Earlier, riot police fired teargas to disperse hundreds of people protesting at the prospect of a life president.
The twice-elected leader who came to power in 1986 is currently barred by the constitution from seeking re-election in polls next year.
A final reading is expected to approve the measure in the next few weeks.
In the capital, Kampala, police had blocked off Constitutional Square where opposition groups planned to meet to march towards parliament.
Organisers then re-convened in another part of town where they began to march towards parliament, chanting anti-Museveni slogans.
"Uganda is destined for doom if Museveni is to be allowed to stand again," read one of the placards.
Mr Museveni introduced the 'Movement' system of government to try and prevent the chaos and ethnic conflicts that plagued Uganda throughout the 1970s and early 80s.
The West is becoming less tolerant of Mr Museveni
While he has defended it as a 'no-party' system, critics say it amounts to one-party rule.
At present political parties are allowed to exist, but candidates for office must run as individuals, not as representatives of a party.
The Ugandan government and opposition parties now all support a return to multi-party politics ahead of elections in a year's time. The constitution is being changed to allow this.
However, Mr Museveni's critics say the changes, which once backed by MPs will need to be approved in a referendum, are also being used as a smoke screen to let the president stay on in office.
The question to be asked in the referendum is: "Do you agree to open up the political space to allow those who wish to join different organisations/ parties to do so to compete for political power?"
Five years ago, a similar referendum backed keeping the "movement" system.