Zimbabwe is to hold presidential and parliamentary elections on 29 March, the government has announced.
President Mugabe is trying to extend his 27 years in office
President Robert Mugabe has been confirmed as his party's candidate.
The opposition is threatening to boycott the elections unless it gets guarantees they will be free and fair at talks with Mr Mugabe's party.
The United States said it regretted the decision, taken without measures "that would have levelled the playing field for all parties".
The Southern African Development Community (SADC) has been trying to promote dialogue between the government and opposition.
But on Wednesday, police used tear gas to disperse a banned march staged by the opposition Movement for Democratic Change (MDC).
The US State Department said the police action was "further illustration of the government of Zimbabwe undermining the spirit of the SADC process".
The MDC said Wednesday's violence went against the spirit of a promise made last December to relax tough security laws, which have been used to hinder opposition campaigns.
MDC leader Morgan Tsvangirai was briefly detained ahead of the march.
"If this is the reaction of this dictatorship, then the elections are a farce," he said.
Recent elections have been characterised by violence against the opposition and accusations of rigging - charges denied by Mr Mugabe.
The MDC and Mr Mugabe's Zanu-PF are holding talks mediated by South Africa and the SADC in an effort to end the political impasse.
The opposition wants constitutional changes to be enacted before the elections, which were due in March.
Both MDC factions have expressed surprise at the announcement.
Wednesday's march was broken up by the police
"It's an act of madness and arrogance," Nelson Chamisa, spokesman fro the Tsvangirai faction, told the AFP news agency.
"At the moment the conditions in Zimbabwe do not allow for free and fair elections and so we are heading towards illegitimacy if we go ahead with the elections," Arthur Mutambara, leader of the other MDC faction, told the BBC's Focus on Africa programme.
Mr Mutambara said a new constitution was needed to ensure a free poll and "pre-empt what has happened in Kenya".
Earlier this month, the BBC was told that former Finance Minister Simba Makoni would challenge Mr Mugabe, either from within Zanu-PF and by launching a break-away party.
Zimbabwe is suffering from an acute economic crisis, which Mr Mugabe's critics blame on his seizure of white-owned farms.
He says he is the victim of an international plot intended to bring him down.
Zimbabwe has the world's highest rate of annual inflation - 8,000% - while just one in five has an official job.