The UN rights chief on Friday denounced violence targeting the opposition by Democratic Republic Congo's security forces, just days before a vote marred by an increasingly tense campaign. "I am deeply worried about the reports of excessive use of force, including live ammunition, by security forces against opposition rallies," the United Nations High Commissioner for Human Rights, Michelle Bachelet, said in a statement.She called on the authorities in Kinshasa to ensure that "the rights to freedom of expression and peaceful assembly - essential conditions for credible elections - are fully protected."Tensions are rising in the vast, chronically unstable country ahead of the December 23 election when voters will choose a successor to President Joseph Kabila.On Thursday, a teenager was shot dead in the central Kasai region as party faithful gathered ahead of a rally by Felix Tshisekedi of the UDPS, one of the leading opposition candidates.And five other opposition supporters were shot dead earlier in the week as they gathered to welcome rival opposition candidate Martin Fayulu as he campaigned in the southeast. "In an already tense electoral environment, I urge the government to send a clear signal that threats and violence against political opponents will not be tolerated," Bachelet said, urging credible probes into all apparent aggressions by the security forces. DRC, a mineral-rich country long plagued by unrest, has never known a peaceful transition of power since independence from Belgium in 1960.