On Monday, several hundred supporters of Kizza Besigye, an opposition leader and presidential candidate, were gathered at a Kampala rugby club awaiting his final rally in the Ugandan capital before Thursday's election when police moved to disperse them.
The police advance and clouds of tear gas sent the throngs of mainly young opposition supporters racing away, with some leaping across a stream to escape. There were no injuries.
Maureen Mbabazi, 22, said, while running away: "What have we done? We don't have any guns. We are just waiting for our leader."
Police said youths were blocking a road and throwing stones.
A senior police officer, said: "It was just hooliganism. We need order here, then the rally can proceed."
The Forum for Democratic Change (FDC) said Besigye was trying to reach the rally but was told by authorities it was not allowed.
Sarah Eperu, an FDC spokeswoman, said: "This was agreed long ago ... They are just cowards."
Tensions before Uganda's vote on Thursday have risen sharply in recent days, particularly after two supporters of Besigye's FDC were shot dead by a soldier last week at another rally.
Besigye, who returned from exile in South Africa last year, seeks to end 20-year rule of Yoweri Museveni, the country's president's. Besigye is trailing by 11 percentage points, according to the latest opinion poll.
Besigye (L) hopes to become
Uganda's new president
Museveni held a rally at a university in Kampala, drawing about 6000 people, witnesses said.
Although police chased away some opposition backers booing members of Museveni's ruling National Resistance Movement (NRM), there was no serious trouble, they said.
Rights groups and Western donors, who fund half of Uganda's budget, are watching the election closely after criticising Museveni for showing an increasingly autocratic streak in the run-up to the vote.
The 62-year-old former rebel leader and son of a cattle-keeper used to be favourite of the West, which hailed him as at the vanguard of a new generation of African leaders.
Western critics, disappointed Museveni was standing for a third term, were angered by the brief jailing of Besigye and the FDC leader's ongoing rape and treason trials.
Besigye was Museveni's doctor during the 1981-86 bush war, then a minister in his government before the pair fell out in the late 1990s.
FDC supporters at Monday's gathering said the police tactics were evidence Museveni and his security forces still acted as if Uganda was a one-party state despite last year's re-introduction of multi-party politics.
"We are tired of this dictatorship. Look how they attack a peaceful gathering. Show the world this"
Peter Wesanga, a supporter of the FDC, said: "We are suffering. We are tired of this dictatorship. Look how they attack a peaceful gathering. Show the world this."
Half a dozen foreign journalists were caught in the melee, which lasted less than an hour before calm was restored.