Four people including a police officer and an electoral official were killed on Sunday in eastern South-Kivu province as the Democratic Republic of Congo voted in presidential elections, the campaign of opposition candidate Felix Tshisekedi said.The officer and official were killed along with two civilians during clashes at a polling station in the Walungu area, campaign director Vital Kamerhe told AFP.He said the violence broke out after the electoral official was accused of trying to rig the vote in favour of Emmanuel Ramazini Shadary, the candidate championed by President Joseph Kabila in power for 17 years."An agitated crowd started fighting with police. An officer was killed, which we deeply regret," said Kamerhe, a former president of the National Assembly who is from South-Kivu.The mob "then attacked the electoral official who died. Two civilians were also killed."South-Kivu authorities said an investigation had been opened into the incident.Catholic Church election observers (Cenco) did not mention the deaths in their third and final report on Sunday's vote at 20:00 GMT.The election gives the DRC the chance for its first peaceful transfer of power since it gained independence from Belgium in 1960.Analysts, though, warned of the threat of upheaval was great, given organisational hitches and suspicion of Kabila, who had refused to quit in 2016 after his two-term limit expired.* Sign up to News24's top Africa news in your inbox: SUBSCRIBE TO THE HELLO AFRICA NEWSLETTERFOLLOW News24 Africa on Twitter and Facebook.