Iran's President Mahmoud Ahmadinejad has arrived in Zimbabwe, despite protests by the MDC party of Prime Minister Morgan Tsvangirai.
The MDC has called Mr Ahmadinejad a "war-monger, a trampler of human rights [and] an executioner".
It has also expressed concern that the visit could affect an attempt to improve relations with the West.
Mr Ahmadinejad was invited by President Robert Mugabe, who has been hit by Western sanctions.
Mr Tsvangirai was due to visit Europe for talks this week on getting the sanctions lifted but his trip was delayed by the volcanic ash flight ban.
Mr Ahmadinejad is due to open a trade fair in the second city Bulawayo on Friday, which the MDC compared to "inviting a mosquito to cure malaria".
"Hob-nobbing with dubious political leaders confirms stereotypes that we are a banana republic," says the hard-hitting statement from the Movement for Democratic Change.
Iran is under a range of UN diplomatic and trade sanctions aimed at stopping it enriching uranium. Iran denies it is planning to make a nuclear weapon.
Zimbabwe is believed to have uranium deposits in the north of the country, but as yet but no exploration contracts have been awarded and the size of the deposits are unknown, according to the Reuters news agency.
Mr Ahmadinejad's government has faced widespread protests in Iran following his disputed re-election in 2009.
However, Foreign Affairs Minister Simbarashe Mumbengegwi, an ally of President Mugabe, said Zimbabwe would benefit from the trip by signing several trade and co-operation agreements with the oil-rich nation.
And the state-owned The Herald newspaper has argued the Iranian president's visit is an effort to strengthen ties between countries targeted by the West.
Western countries have labelled Iran part of an "axis of evil" and Zimbabwe a pariah state, says the paper.
"The West's neo-colonial agenda should only make us stronger," it added.
After Zimbabwe, Mr Ahmadinejad is due to visit Uganda, where oil has recently been discovered.
The MDC joined a power-sharing government in 2009 in an attempt to revive the ruined economy.
Since the European Union and the US imposed a travel ban and assets freeze on Mr Mugabe and his close allies, he has tried to improve relations with other nations, such as China and Malaysia.
Mr Mugabe previously tried to portray the MDC as a stooge of the former colonial power, the UK.
He has criticised it recently for failing to get the sanctions on him lifted.
They were imposed after the US and the EU accused Mr Mugabe of rigging elections.
He says they were really a punishment for his policy of seizing land from white farmers.