Police fired tear gas and water cannon at stone-throwing crowds protesting in central Nairobi against an election oversight body that they say is biased and should be scrapped, a Reuters witness reported. Officers armed with batons confronted hundreds of protesters outside the offices of the Independent Electoral and Boundaries Commission (IEBC), the third clash over the issue in less than a month.Kenya does not hold its next presidential and parliamentary polls until August 2017, but politicians are already trying to galvanise their supporters in a nation where violence erupted after the 2007 vote and the opposition disputed the 2013 result.The opposition CORD coalition, led by Raila Odinga who lost the 2013 vote and unsuccessfully challenged the result in court, has accused the IEBC of bias and said its members should quit. IEBC officials have dismissed the charge and say they will stay."For free and fair election, IEBC must go," read a banner held aloft by one demonstrator on Monday.As numbers grew, police fired tear gas and water cannon from trucks parked nearby at protesters. A Reuters witness saw one protester carrying a bag of stones, while others threw them at police ranks. Police struck some protesters with batons.Last week, police fired tear gas and water cannon at hundreds of protesters, some of whom threw stones. Police also used tear gas to disperse a protest last month.President Uhuru Kenyatta, who is expected to seek re-election next year for a second and final term, has urged opponents not to take to the streets.Despite the 2013 legal challenge, that vote proceeded smoothly and Odinga accepted the court ruling in a nation where ethnic loyalties usually trump policy among voters.After the disputed 2007 vote, about 1,200 were killed in ethnic fighting. Western diplomats have urged the authorities to work carefully with citizens to ensure peaceful elections in 2017. -Reuters..