At least one person was killed in Uganda on Monday as police and opposition supporters clashed in the capital days ahead of presidential polls, police said.

"Police can confirm one person died during the confusion today," Kampala police spokesman Patrick Onyango told AFP, without giving further details.

Opposition politicians said three people had been shot dead, but the claims could not be independently verified.

"Three of our supporters have been shot dead and several other people have been seriously injured and are in hospital," said Mubarak Munyagwa, who is hoping to be mayor in Kampala for the opposition Forum for Democratic Change (FDC) party.

Police fired tear gas to disperse opposition supporters as they briefly detained a top presidential challenger, FDC leader Kizza Besigye.

Seven opposition candidates are vying to deny veteran leader President Yoweri Museveni a fifth term at the February 18 election, and there are fears violence could mar the campaign, with all sides accusing each other of arming militias to press their claims to power.

Museveni faces his stiffest challenge yet from Besigye, his former personal doctor, and Amama Mbabazi, a former prime minister and ruling party stalwart now running as an independent.