Uganda's President Yoweri Museveni has all but extended his 30-year rule after taking a strong lead in general elections disputed by rivals.

With about half the votes counted on Saturday, the incumbent candidate had 60 percent of votes to leading challenger Kizza Besigye's 35 percent.

Full results of the vote are expected to be released on Saturday afternoon.

The vote has been marked by allegations of ballot fraud and multiple arrests of opposition activists, including Besigye.

Police have arrested Besigye four times since the day of election and the opposition candidate is currently detained at his house in the capital Kampala.

Besigye's third arrest was caught by Al Jazeera cameras as he tried to access a housewhere ballots were suspected of being altered.

A Ugandan soldier points a heavy-calibre machine gun in the direction of supporters of opposition leader Kizza Besigye [AP/Ben Curtis]

On Friday Police in riot gear had set off stun grenades and fired teargas at Besigye supporters, who responded by hurling rocks and erecting street barricades.

US Secretary of State John Kerry called Museveni to voice concern over Besigye's detention, harassment of opposition figures and the shutdown of social media such as Facebook and Twitter.

Police officials said they were at Besigye's home as a preventative measure to prevent a further escalation of violence and denied detaining him.

'Lacking transparency'

Al Jazeera's Malcolm Webb, reporting from Kampala, said the conduct of the election had been criticised by EU observers.

"They said the use of force against the opposition has been unacceptable and they also said the electoral commission has lacked transparency," Webb said.

The electoral commission rejected the criticism and said it had conducted the elections in a free and fair way.

Earlier, Jonathan Taremwa, a spokesman for the electoral commission, told Al Jazeera the vote was "transparent" and "fair".

"Some people didn't get to vote. It was unfortunate, it was regrettable, and the commission offered an apology. We finally had stations [affected by delays] opened for votes and later extended the voting from 4pm to 7pm," Taremwa said.

Besigye's supporters said the delays were deliberate and were aimed at favouring Museveni, whose rival is popular in Kampala.

Museveni, 71, a former rebel who seized power in 1986, is widely expected to win a fifth term, which would extend his power into a fourth decade.

Besigye was Museveni's field doctor during the war which brought him to power, and served as deputy interior minister in his first cabinet.
He broke ranks with the president in 1999, saying Museveni was no longer a democrat.

Source: Agencies And Al Jazeera