A teachers' union in Zimbabwe has threatened to call a strike unless government supporters stop attacks on its members. The Progressive Teachers Union said 133 of its members had been assaulted, and more than 1,700 had fled amid threats. Many teachers worked as polling officers during the disputed presidential election on 29 March. The opposition won the most votes in the poll, but is still debating whether to contest a second-round run-off. The main opposition party, the Movement for Democratic Change (MDC), says the government delayed giving the results for more than a month so it could carry out a campaign of violence against opposition supporters ahead of a run-off. The Progressive Teachers Union says it is not political, but its members have been accused of helping the opposition during the election in March. The union's Raymond Majongwe told the BBC that some of the attacks on teachers had been savage. "They were beaten with iron bars, some have had their legs and limbs and hands seriously injured," he said. "Quite a lot have been hit on the head and its quite tragic, it's terrible." The BBC's Will Ross in Johannesburg says this is not the first time teachers in Zimbabwe have been targeted. During parliamentary elections in 2000, a government minister accused them of polluting the minds of young children, prompting government supporters to drag teachers from their classrooms and beat them in front of their students. Our correspondent adds that Zimbabwe's education system was once one of the finest in Africa and teachers commanded a great deal of respect, but many have left as the situation in the country has deteriorated. Results published by Zimbabwe's electoral commission last week gave MDC leader Morgan Tsvangirai more votes than President Robert Mugabe, but not the 50% needed to avoid a run-off. Mr Mugabe has said he will stand in a run-off, but a date for the second round of voting has yet to be set. Mr Mugabe would win automatically if his rival refused to take part. On Sunday, Mr Tsvangirai's MDC said it was discussing the conditions under which it could take part in a second round.