Harare - Suspected ruling party supporters burnt a vehicle belonging to Zimbabwe's main opposition MDC party in apparent retaliation for a demonstration it held in Harare, a top party official said on Thursday.Meanwhile at least six MDC supporters were arrested and another 10 injured by police armed with baton sticks during Wednesday’s protest march in central Harare.Beaten with baton sticks"Ten were injured and received treatment (at a Harare clinic). Three were seriously injured and I think they were admitted. They were beaten up with baton sticks," MDC Secretary General Douglas Mwonzora told News24.He said a vehicle belonging to the MDC Youth Assembly, which organised Wednesday’s march, was torched overnight in Harare’s Kuwadzana suburb by suspected state agents.'Too frightened to talk'"What these people did is that they followed the vehicle to where it is parked and they burnt it overnight," he said."The security which is supposed to be in charge of the car park are not talking at all. They say they are so frightened to talk."On Wednesday a group of MDC activists tried to march through central Harare to press for electoral reforms ahead of the 2018 vote, in which President Robert Mugabe will seek re-election.'Gravely concerned'Riot police broke up the march soon after it started, firing tear gas and water cannons that also affected ordinary people going about their business.The Zimbabwe Human Rights Association (ZimRights) says it is "gravely concerned" by the rise in political conflict that led to the burning of the MDC vehicle."Given that Zimbabwe has previously witnessed worse election-related violence, these alarming incidences and political developments should be taken seriously by the international community to avert a slide into more violence," it said in a statement.