The election, seen as the key test of the long and bloody US-led military campaign in Afghanistan, is scheduled to choose a successor to President Hamid Karzai on April 5.
But there is no front-runner in the race, and international donors fear that a repeat of the chaotic and fraud-ridden 2009 election could wreck any claims of progress in establishing a stable, functioning Afghan state.
Key opposition figures in the alliance announced today include 2009 runner-up Abdullah Abdullah, warlord-turned-governor Atta Mohammad Noor and former communist general Abdul Rashid Dostum.
The group, calling itself the "Afghanistan Electoral Alliance", is based on the Northern Alliance coalition that overthrew the Taliban with US help in 2001 after the 9/11 attacks.
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Ethnically dominated by Tajik, Uzbek and Hazara parties, it will struggle to win the election without the support of Pashtuns, the largest and traditionally most politically powerful ethnic group in Afghanistan.
Earlier this week, Karzai said he would work to ensure a transparent and credible election but he expressed concern that too many candidates could make the race confusing for voters.
Other potential runners include Qayum Karzai, the president's brother, Omar Daudzai, the ambassador to Pakistan, and former interior minister Ali Ahmad Jalali.
Taliban leader Mullah Omar last month dismissed the elections as "a waste of time". The militants have vowed to step up attacks before the withdrawal of NATO-led coalition forces by the end of next year.
Candidate nominations for the election open on September 16.