The Coalition of Forces for Democratic Change (CFCD), which groups parties that opposed Taya's rule and won about 40 per cent of parliamentary seats in legislative elections last November and December, accused the military leaders of being biased.
In a letter to the African Union, European Union and other international organisations, the CFCD said it "vigorously denounces the blatant undermining of the transition authorities' neutrality".
The military rulers have gained international support by promising free and fair elections and pledging that their leader, Ely Ould Mohamed Vall, will not run.
The CFCD's letter, which was also addressed to the Arab League, Organisation of the Islamic Conference and the French-language grouping Francophonie, said the military were "now running an open campaign in favour of one candidate".
"This campaign essentially consists of the authorities summoning influential people, businessmen and state officials to ask them clearly to support a single candidate in the presidential election, and warn them against supporting any other candidate," the letter said.
|"This campaign essentially consists of the authorities summoning influential people ... to support a single candidate in the presidential election"|
Statement by the Coalition of Forces for Democratic Change
It did not name the candidate in question.
Political analysts say the military would favour Sidi Ould Cheikh Abdallahi, a member of a traditionalist tribe of marabouts, or religious leaders, who has held several ministerial positions including the economy portfolio under previous administrations.
They say Abdallahi, hailing from the centre of the country, could gain support from the Arab north and black African south of Mauritania, and would be less likely than opposition politicians to investigate abuses by previous administrations, thus protecting military figures who might be implicated.
The coalition accused the military of using public appointments and economic incentives to support its chosen candidate, and in a separate letter to the current administration, it asked Vall to refrain from appointing any state officials before the election unless absolutely necessary.
Vall was overseas on Sunday and government officials were unavailable for comment.
About 10 candidates have said they will stand in March, including former ministers such as Abdallahi, a former central bank governor, a former military ruler and leaders of political parties including at least two from the CFCD coalition.