Although he made no public comment after the ECC announcement, Karzai did pledge to "fully respect the constitutional order" in talks with UN chief Ban Ki-moon, according to a UN spokeswoman.
US Secretary of State Hillary Clinton said she expected the president to reveal his intentions on Tuesday, voicing hope of a resolution "in the next several days."
"We're looking to hear from President Karzai tomorrow (Tuesday)," Clinton said.
"He is going to announce his intentions. I am going to let him do that but I am encouraged at the direction that the situation is moving," she added.
Afghanistan's ECC said it found "clear and convincing evidence" of fraud, including entire ballot boxes with papers filled in with the same pen or same mark.
An independent US monitor said neither candidate had won outright, meaning Afghanistan could face a second round between Karzai and his main rival Abdullah Abdullah, although the two are expected to make a deal.
The ECC said it ordered the Independent Election Commission (IEC) -- the final arbiter of the election results -- "to invalidate a certain percentage of each candidate's votes" and results from 210 polling stations.
The ECC findings, which it said were "final and binding", could see the IEC forced to call a run-off between Karzai and Abdullah.
Karzai, against whom most of the ballot-stuffing allegations were made, leads preliminary results with about 55 percent of the vote, with Abdullah on around 28 percent.
The ECC refused to divulge corrected results, but one Western diplomat and the respected election monitor said Karzai's share of the vote had now fallen to about 48 percent -- not enough for outright victory.
US-based Democracy International said around one million votes cast for Karzai have been deemed invalid and adjusted figures gave the president 48.29 percent of the overall vote, with Abdullah now on 31.5 percent.
This "should necessitate a run-off election between Hamid Karzai, and the second-place candidate, Dr. Abdullah Abdullah," Democracy International said.
A total of 1.3 million votes cast were invalid, it added, based on the ECC figures, or around a quarter of the total 5.6 million votes cast.
Faced with fears that the IEC -- which Abdullah and others have accused of being pro-Karzai -- may not accept the ECC findings, there were calls for quick action to end the political deadlock.
The UN spokesman in Kabul, Aleem Siddique, said the IEC should "move swiftly to announce either a final certified result or the requirement for a second round".
Former foreign minister Abdullah told US National Public Radio a run-off election would restore faith in the democratic process.
Meanwhile Peter Galbraith, a US diplomat sacked by the UN last month after alleging election fraud, told AFP the extent of cheating was even greater than reported.
The ECC announcement comes as patience among Afghanistan's international backers wears thin.
Karzai's survival -- and that of his government -- depends on the continued support of the international community, which has poured billions of dollars in aid into the country since toppling the Taliban in 2001.
Abdullah supporters have said they will not accept a Karzai victory because of the vote-rigging claims, raising fears about the next administration's legitimacy at a time of spiralling insurgent violence.
Political analyst Waheed Mujdah said even if a second round is called, Abdullah could be prepared to do a deal.
US President Barack Obama is considering a request from his military commanders for another 40,000 troops to boost the more than 100,000 already in Afghanistan under US and NATO command.
But White House chief of staff Rahm Emanuel and Senator John Kerry both said at the weekend that it would be "irresponsible" to send more troops to Afghanistan to fight the resurgent Taliban before the election is resolved.
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