The electoral commission released Friday's results in under 48 hours, compared to five weeks for the first poll held on March 29.
Mugabe, who has ruled Zimbabwe since its independence in 1980, was the sole candidate and won 85 per cent of the vote.
The opposition Movement for Democratic Change (MDC) led by Morgan Tsvangirai boycotted the poll due to intimidation and violence.
In power since independence from Britain in 1980, Mugabe was quickly sworn in for a sixth term in a ceremony on the lawns of State House.
The veteran leader invited Tsvangirai to attend his inauguration in what the government said was a gesture of "political engagement".
But Nelson Chamisa, the spokesman for the opposition MDC, told Al Jazeera on Sunday that Zanu-PF and Mugabe had stolen the election and that Tsvangirai would not attend the inauguration of president Mugabe.
|Pan African Parliament observers have called for fresh polls [AFP]
"How do you expect Tsvangirai to attend a ceremony where he is supposed to be the one inaugurated?" he said.
"It is clear that Robert Mugabe cannot win any election other than the one where he is contesting alone - like this one, which is a sham."
Al Jazeera's Mike Hanna, reporting from Johannesburg, in neighbouring South Africa, said there was a shift in the opinion of many African leaders on Zimbabwe.
"There is a greater willingness to make public their [African leaders'] concern about what is happening in Zimbabwe and make public their criticism of Mugabe," he said.
"In recent weeks a large number of African leaders have broken their silence and in particular and most importantly we have heard from African states that have been very close allies of Mugabe in the past such as neighbouring Angola, where president Eduardo dos Santos was the first to break the silence.
"We have heard a similar message from Zambia and the prime minister of Kenya go so far as to suggest that African troops should be sent to Zimbabwe."
Zimbabwe's one-man presidential election was branded a farce by many and prompted calls for the African Union to shun Mugabe at its meeting in the Egyptian resort of Sharm el-Sheikh on Monday.
The MDC appealed for the AU not to welcome Mugabe at the meeting and Thokozani Khupe, the MDC vice president, said she had no plans to talk to the Zimbabwean delegation on the sidelines of the summit.
International criticism of Mugabe has grown since the one-man election and African nations have also begun to condemn the Zimbabwean leader.
|MDC voters have been attacked by Zanu-PF militia said to be on the rampage
A group of African politicians who observed the election called the atmosphere in Zimbabwe during Friday's vote as "tense, hostile and volatile" and called for a new round of polling.
Human Rights Watch said it had documented numerous incidents of voters being coerced into taking part in the run-off election and of others being attacked after polling was completed.
The US-based group also called on African leaders to impose sanctions against Zimbabwe's government and refuse to recognise the legitimacy of Mugabe's victory.
The US and a number of European powers pushed for a UN security council resolution that would have stated the results "could have no credibility or legitimacy", but South Africa blocked the move, arguing the Security Council was not in the business of certifying elections.