The election, due to take place on April 5, will mark Afghanistan's first ever democratic transfer of power as Karzai steps aside after leading the country since the Taliban hardline regime was ousted in 2001.
The presidential race, from which Karzai is barred after serving two terms, is seen as the key test of 13 years of bloody international military intervention and billions of dollars of aid.
"So far, 27 people -- both individuals and members of parties and coalitions -- have collected registration forms for the presidential election," Independent Election Commission (IEC) spokesman Noor Mohammad Noor told AFP.
Presidential hopefuls must be aged at least 40, not convicted of any crimes, provide 100,000 voter cards to prove they have a network of supporters, and also lodge a deposit of one million Afghanis (USD 18,000).
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The tough new criteria were designed "to limit the number of runners", Noor said, after around 40 candidates stood in the chaotic 2009 election that was marred by massive fraud.
Karzai, a charismatic and mercurial figure, has dominated Afghan politics as the country has struggled to develop amid a fierce Taliban insurgency, and there are no obvious front-runners to succeed him.
"Credible, inclusive, and transparent presidential and provincial elections in 2014 are now the most important element of Afghanistan's transition," US ambassador James Cunningham wrote in an article in the local media last week.
He called for "broad participation in the elections by men and women from all segments of Afghan society" -- after low participation by women and Pashtuns, the country's largest ethnic group, in the 2009 vote.
Many Afghans fear a surge in violence during campaigning and on polling day, but the government has said its security forces will ensure a safe environment for the election.
With political manoeuvring well under way in Kabul, attention has now focused on low-profile foreign minister Zalmai Rassoul as a strong technocrat candidate from within Karzai's camp.
Other potential runners include Qayum Karzai, the president's brother, and former interior minister Ali Ahmad Jalali.
The election next year coincides with the withdrawal of 87,000 NATO combat troops as Afghan soldiers and police take charge of the battle against the Taliban.