KAMPALA - Ugandan police fired tear gas on Monday to disperse opposition supporters as they briefly detained a top presidential challenger, Kizza Besigye, days ahead of presidential polls.

Besigye, a three-time failed presidential candidate who heads the Forum for Democratic Change (FDC) party, was held after "chaotic scenes" as he campaigned in the capital Kampala ahead of Thursday's election, the state-run New Vision newspaper reported.

"They were firing tear gas, a lot of tear gas," said FDC supporter Swaibu Mugalu, 32, describing how police blocked Besigye from holding a rally in central Kampala.

Besigye was driven to a police station, before then being taken home by police, with police spokesman Patrick Onyanyo stressing he was "not under any form of arrest."

FDC spokesman Semujju Nganda claimed "several" supporters were injured during scuffles with police. "We protest in the strongest terms police brutality towards our supporters and targeting our candidate," he said.

Minister for the Presidency Frank Tumwebaze however said it was "madness" that Besigye had tried to hold a rally in the centre of the capital, saying the police had a duty to ensure calm.

He accused Besigye of "seeking publicity... after sensing defeat ahead."

Besigye has been repeatedly arrested in the past, and is commonly freed without charge shortly after. He called for calm after his release. "I appeal to my supporters to be calm. Let us remain firm and law abiding. We shall assert our rights," Besigye said, according to the New Vision.

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.

The US State Department on Monday stressed the need for a "peaceful, transparent and credible electoral process" and called on all sides to "refrain from provocative actions or rhetoric that raise tensions."

Government spokesman Ofwono Opondo earlier this month claimed opposition parties were raising militia to disrupt the election, claims they have denied.

"Violence or threats of violence from any group or individual are unacceptable, and those who participate in such acts - regardless of which candidate they support - must be held accountable," the US statement added.

"We strongly urge the government and electoral authorities to ensure a level playing field and transparent process, including through fair application of the law, so that all candidates have an equal opportunity to express their views and voters have the opportunity to hear them."

Museveni, who seized power in 1986, is one of Africa's longest serving leaders, after Equatorial Guinea's President Theodore Obiang Nguema, Angola's Jose Eduardo Dos Santos, Zimbabwe's President Robert Mugabe and Cameroon's Paul Biya.

Aged at least 71, Museveni is widely expected to be re-elected, having changed the constitution in 2005 to allow him to run again.