Zimbabwe s government has extended the deadline by which the presidential run-off vote must be held to 31 July.
The main opposition Movement for Democratic Change (MDC) said the decision was illegal and unfair.
Opposition leader Morgan Tsvangirai beat President Robert Mugabe in the first round on 29 March, but with not enough votes to avoid a run-off.
Meanwhile the justice minister has proposed establishing cross-party teams to probe acts of political violence.
"Whenever there is a claim of an act of politically motivated violence committed, it should be very good that we form joint teams made up of the (ruling party) Zanu-PF and MDC so that we can establish the veracity of these claims," Justice Minister Patrick Chinamasa told state television.
The MDC had threatened to boycott the second round of voting after accusing Zanu-PF of trying to rig it but over the weekend Mr Tsvangirai said he would take part.
He has been in neighbouring countries since the first round because of alleged threats to his life.
Bu the MDC says that Mr Tsvangirai will return to address a rally in Bulawayo on Sunday.
Business as usual
According to a special government gazette published on Wednesday, Mr Chinamasa officially extended the period for the second round of voting.
He said that instead of 21 days, the Zimbabwe Electoral Commission (Zec) now had 90 days from the day the results were announced on 2 May to hold the vote.
This means the election should now take place on, or before, 31 July.
The MDC has always demanded that the poll take place within the originally mandated time frame.
But Zec Chairman George Chiweshe told the BBC News website that the previous deadline of 23 May left "insufficient time" for preparations.
The MDC has accused the ruling Zanu-PF party of beating and torturing MDC supporters in an attempt to either keep them away from the polls or intimidate them into voting for the ruling party.
"It is part of a programme to give Mugabe and Zanu-PF time to torment and continue a campaign of violence on the MDC," MDC spokesman Nelson Chamisa told Reuters news agency.
But Deputy Information Minister Bright Matonga said that under the Electoral Act there was leeway to delay the election for up to 12 months for logistical reasons.
"We are very eager as Zanu-PF and also as the government to have these elections done as quickly as possible so we can get on with our lives," he told the BBC s Focus on Africa programme.
He said the reports of electoral violence were exaggerated.
"In Zimbabwe it s business as usual, you wouldn t think when you watch on BBC or CNN that this is the same Zimbabwe that people are talking about - it s very peaceful."
The MDC says Mr Tsvangirai s campaign will start on Sunday.
"After the White City rally President Tsvangirai will take his tour across country and will visit every village and town as he thanks the people of Zimbabwe for voting for the MDC and its president," the MDC said in a statement.