The move means ZBC will carry no more MDC campaign advertisements
Zimbabwe's public broadcaster ZBC has said it will no longer carry campaign adverts from the opposition party ahead of next week's presidential election.
The Movement for Democratic Change said it would appeal against the decision.
Justice Minister Patrick Chinamasa defended the move saying international coverage favoured the MDC and never reported the ruling Zanu-PF's position.
Earlier, UN Secretary General Ban Ki-moon expressed concern over the political violence in Zimbabwe.
Adding his voice to growing international concern, he said the violence in Zimbabwe could undermine the outcome of the 27 June run-off vote.
"Violence, intimidation and the arrest of opposition leaders are not conducive to credible elections," he told the UN General Assembly in New York.
The MDC says 66 of its supporters have been killed and 25,000 forced to flee their homes in a state-sponsored campaign of violence.
Correspondents say the ban on adverts will not make a great deal of difference, as news bulletins at the state-run ZBC have always favoured Mr Mugabe, only mentioning the opposition in negative terms.
There are no privately controlled radio or TV stations in Zimbabwe and only a few weekly newspapers, which most people cannot afford.
US Secretary of State Condoleezza Rice is due to chair an informal UN Security Council meeting on Zimbabwe later on Thursday, in an attempt to maintain international political pressure.
On Wednesday, South African President Thabo Mbeki spent his 66th birthday continuing his efforts to mediate between President Robert Mugabe and the opposition leader, Morgan Tsvangirai.
He held separate talks with both presidential candidates as pressure mounted on Mr Mugabe to curtail political violence ahead of the poll, but released no statement on the talks.
The MDC has criticised Mr Mbeki's policy of "quiet diplomacy" for failing to hold Mr Mugabe to account.
Official results show Mr Tsvangirai, leader of the Movement for Democratic Change (MDC), gained the most votes in the first round of the presidential election in March but did not pass the 50% threshold for outright victory.
Haile Menkerios (R) discussed the political stand-off with Mr Mugabe
A senior UN official, Haile Menkerios, earlier met President Mugabe to discuss the political stand-off and what the UN says is the increased suffering of an already vulnerable population.
The UN is prepared to pay to fund election monitors to oversee the run-off vote.
South Africa is opposed to the Security Council having too much involvement, the BBC's Laura Trevelyan reports from the UN.
Pretoria argues that it is not for the council to resolve disputed elections.
Earlier, an African poll observer warned that he would not endorse the vote if current levels of violence continued.
Marwick Khumalo, head of the Pan-African Parliamentary observers, told the BBC his team had received horrendous reports of attacks and that the political environment was not conducive to a free poll.
Thousands of opposition supporters have been beaten or worse
But with the vote just days away, there is a growing sense of urgency with political violence beginning to spread from the countryside to the towns, says the BBC's Peter Biles in Johannesburg.
Mr Mugabe has been waging a fierce campaign to extend his 28-year rule since Mr Tsvangirai failed to win enough votes to score an outright victory in March's disputed first round.
Kenyan Prime Minister Raila Odinga has called for an international peacekeeping force to be deployed in Zimbabwe to ensure a free and fair vote.
"It is time for the leaders of Africa to say to President Mugabe that the people of Zimbabwe deserve a free and fair election," he said.
Rwanda's President Paul Kagame has also criticised Mr Mugabe, asking why he bothers holding an election, if he says he will not respect the outcome, reports the Reuters news agency.
British Prime Minister Gordon Brown says he has spoken to the leader of South Africa's governing African National Congress, Jacob Zuma, about the possibility of deploying 1,000 election observers from the ANC.
Western observers have been banned, as the government accuses them of being biased in favour of the opposition.
The government has also said it wants to reduce the number of local election monitors, after 50,000 asked for accreditation.