KAMPALA (AFP) - Ugandan police briefly detained top presidential challenger Kizza Besigye after a day of polling Thursday marred by long delays and police firing tear gas to disperse furious voters in the capital Kampala.

Besigye's Forum for Democratic Change (FDC) party said he was held after checking on reports of an illegal tally centre in his home district manned by the ruling National Resistance Movement party.

"He walked in and saw them pre-ticking ballot papers. They arrested him and took him to an unknown place," said senior FDC official Shawn Mubiru.

Besigye's lawyer, Ladislaus Rwakafuzi, later said he had been "released without charge and taken home," in line with a past pattern of detention and release.

Kampala police spokesman Patrick Onyango told AFP Besigye "has not been arrested. He is a free man".

"What happened is that Besigye stormed a police facility where we operate and receive emergency calls... Besigye wanted to force his way inside the facility with media and ended up assaulting a police officer," Onyango said.

"The officer has opened up a case of assault against Besigye and we shall open a case of criminal trespass against Besigye because he wanted to enter without permission".

The voting, for the presidency and parliament, sees incumbent head of state Yoweri Museveni facing a challenge from seven candidates.

His biggest rival is Besigye, 59, who is making his fourth run at the presidency.

But electors in parts of the capital - where support for the opposition is traditionally strong - were unable to cast their ballot for several hours as ballot boxes and papers did not arrive on time.

The delay sparked concern from an independent watchdog and anger among voters.

"A delay of an hour or two is excusable. Delays of three, four, five and even six hours, especially in Kampala, are absolutely inexcusable and will not inspire trust and confidence in the system and the process," Olusegun Obasanjo, the head of the Commonwealth Observer Group in Uganda, told AFP.

Ballot counting began even while others still queued to vote in the capital, with polls closing around nightfall.

"People are quite angry and everybody is believing that there is something wrong behind this because of the way they are delaying things," said Moses Omony, a motorbike taxi driver.

"We know this has been done intentionally," said Marius Nkata, a builder.

The election commission earlier said it "regrets" delays in some areas and appealed for calm, but opposition leader Amama Mbabazi, a former prime minister and ruling party stalwart, condemned the body.

"We fear that developments are intentional and deliberate, as there are no justifiable reasons to explain delays," read a statement released by Mbabazi's chief of staff, Solome Nakaweesi.

"The fact that voting began so late and some voters may be disenfranchised, questions whether this will be a credible and meaningful election."

It added the party was "disturbed by the failure of the Electoral Commission to act timely and decisively in the face of these irregularities", including the delays in the wider Kampala district, as well as of reports of "pre-ticked ballots" in favour of Museveni and candidates from his NRM party.

In turn, the NRM said claims of pre-ticked ballots were "unsubstantiated and without foundation", while calling the number of stations registering delays "isolated."

Museveni is widely predicted to win a fifth term, with the ex-rebel fighter who seized power in 1986 entering his fourth decade in power.

After casting his vote in the west of the country the 71-year old incumbent said he was going to have a rest. "I have not been sleeping," he said.

"Tomorrow I will go for my cross-country walk to exercise and then go to my cows."

He added that anyone threatening election violence would be "put in the freezer" to cool down.

Social media, including Facebook and Twitter, were largely inaccessible on voting day although Internet-savvy Ugandans dodged the apparent shutdown using virtual private networks.

Government regulator, the Uganda Communications Commission, said the attempted shutdown was for "security reasons" without giving details.

Over 15 million Ugandans were registered to vote, casting ballots in more than 28,000 polling stations for both a president and members of parliament, with 290 seats being contested by candidates from 29 political parties.

More than 150,000 police, soldiers and other security forces have been deployed to ensure tight security, according to election officials.

Initial results are expected as early as Saturday afternoon with the leading candidate requiring more than 50 per cent of votes cast to avoid a second round run-off.