Ugandans vote Thursday in a highly-charged election after one of the bloodiest campaigns in years, as veteran leader Yoweri Museveni seeks a sixth term against a singer-turned-lawmaker half his age.

The World Health Organization emergency committee will decide Friday whether to declare a raging Ebola epidemic an international threat, after an outbreak that began in Democratic Republic of Congo crossed into Uganda.

The WHO panel, which was formed in 2005, has used the label "public health emergency of international concern" for only four previous epidemics.

Uganda has sent an official complaint to neighboring Democratic Republic of Congo after four police officers were shot dead on a lake between the neighbors, the foreign ministry said Tuesday.

The Ugandan police officers were in a patrol boat investigating reports of illegal fishing on Lake Albert on Saturday when gunmen opened fire. Uganda has blamed DR Congo troops for the killing.

Gunmen killed four Ugandan policemen on a lake bordering Democratic Republic of Congo, an officer said Monday, blaming Congolese troops for the shooting.

The Ugandan police officers were in a patrol boat investigating reports of illegal fishing on Lake Albert on Saturday when gunmen opened fire.

Gunmen in northern Uganda have abducted 20 villagers close to the border with South Sudan, Ugandan police said Friday, expressing fears they were captured by a militia from their war-torn neighbor.

The rare attack took place in the early hours of Tuesday in the Yumbe district of Uganda's northern West Nile region, which lies close to the porous border with South Sudan.

War crimes judges Wednesday confirmed 70 charges against notorious Lord's Resistance Army commander Dominic Ongwen for crimes committed in Uganda, including keeping sex slaves and recruiting child soldiers.

Judges at the International Criminal Court "confirmed 70 charges brought by the prosecutor against Dominic Ongwen," the Hague-based court said, paving the way to a trial on the alleged crimes.

"In the past you could get away with imprisoning and killing everyone," said Ugandan lawyer and poet Kabumba Busingye. "Now it has become much more difficult to be a dictator."

Under Idi Amin, 1970s Uganda was a rough dictatorship ruled with brutal buffoonery, but Yoweri Museveni, a former rebel who helped overthrow Amin and his successor to seize power in 1986, is a "sophisticated dictator in a suit" said Busingye.

The U.N.'s rights office voiced concern Tuesday over rising tensions in Uganda following the arrest of opposition leaders and intimidation by the security forces during the country's presidential election.

Uganda's long-serving leader Yoweri Museveni was declared winner of the February 18 vote with 60 percent support against 35 percent won by his main challenger, Kizza Besigye, who has rejected the results.

Ugandan police arrested 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 detained after checking on reports of an illegal tally center in his home district manned by the ruling National Resistance Movement party.

Ugandan election officials on Wednesday said they were expecting presidential and parliamentary polls to pass off peacefully, a day before seven candidates challenge veteran leader Yoweri Museveni's three-decade grip on power.

"The stage is set. We have dispatched electoral materials to all polling stations throughout the country and are ready to kick off the exercise," national electoral commission spokesman Jotham Taremwa told AFP.