Geneva - The UN's rights office voiced concern on 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% support against 35% won by his main challenger, Kizza Besigye, who has rejected the results. "We are concerned by the tense post-electoral situation in Uganda," a rights office statement said, noting the heavy military and police deployments around the capital Kampala. Two people were killed as police were dispersing election-related crowds in separate incidents on February 15 and 19, the rights office said. Besigye, who has challenged Museveni in four consecutive elections, was arrested three times during and after the election and was being held at a police station outside Kampala. Two other opposition leaders have reportedly been arrested, while another, opposition candidate Amama Mbabazi - once one of Museveni's staunchest allies - has been under house arrest since Saturday, the UN statement said. The rights office reminded the government in the East African nation that it had a duty under international law to inform suspects of the reason for their arrest and promptly notify them of possible criminal charges. It also raised concern "about the intimidating display of force used on Friday by Ugandan police and military forces to evacuate the FDC headquarters in Kampala," referring to Besigye's Forum for Democratic Change party. The UN said there were reports of security forces using tear gas and live ammunition to clean out the FDC building.International and domestic observers have criticised the election which extended 71-year-old Museveni's rule into its fourth decade.