"When there are problems, how can we hold an election? Even if half of the population wants to vote, they won't be able to. How will [that] be a legitimate election?" he said on Monday.
"We need 150,000 troops for security ... who will provide that?"
Karzai has said he will make a statement on Tuesday regarding the request for a change to the election date, but it was not clear whether he would be altering his position.
The United States and other Nato members have voiced concern over the possibility of moving up the election.
"Elections in August, as proposed by the Independent Elections Commission, [are] the best means to assure every Afghan citizen would be able to express his or her political preference in a secure environment," the US state department said in a statement on Sunday.
Barack Obama, the US president, has ordered 17,000 more troops to deploy to Afghanistan, but the increased numbers would likely not be in the country if elections were to be held in April or May.
Supporters of Karzai say that the Afghan president has done his job in examining the constitution and applying an election within its terms.
The president's opponents, however, have accused him of attempting to gain an unfair advantage by trying to alter the election date.
The main opposition alliance, which comprises about 20 political groups, said Karzai may have issued his decree in an attempt to force a state of emergency that would see his term extended until the next election.
Sayed Aqa Fazil Sancharaki, a National Front spokesman, said the opposition would still take part in an election if it were to be held in April.
On the streets of Kabul, Afghans told Al Jazeera it did not matter whether elections were held in August or April, saying they didn't believe the politicians were working for their benefit.
"Whoever is the next president will only work to feed himself. He will forget about the poor Afghans and make money for himself," Qand Agha, a Kabul resident, said.
"Karzai already fed himself. In that regard he may be the better choice."
Another Kabul resident, Baba Qari, said Afghan politicians "just make promises that they will bring peace and create jobs ... when they become president they forget about all those promises".
"They are all the same donkey with the same attitude," he said, referring to a common Dari expression.