Elections in 2008 led to severe bloodshed following allegations of malpractice and fraud.
The strongman's seventh successive win has enraged the opposition. The Movement for Democratic Change (MDC) led by the losing candidate, Morgan Tsvangirai, plans to hold mass rallies across Zimbabwe claiming foul play.
The opposition has vowed to take legal action against Mugabe, who has been ruling Zimbabwe for 33 years, and boycott state institutions.
"Once this is resolved, a free and fair election must be held soon. We did not lose this election. We have won it. This is not a personal issue, it is a national issue. The fraudulent and stolen election has plunged Zimbabwe into a constitutional, political and economic crisis," said the opposition leader.
However, Tsvangirai vowed to resolve the crisis through "peaceful and democratic" means.
The Zimbabwean police have also urged politicians not to incite violence.
"We want to warn politicians that are considering this option of inciting Zimbabweans into mass protest that it is not good for the country. Politicians should not blame the police when they find themselves on the wrong side of the law," said chief police spokesperson Charamba.
The African Union (AU) and the regional African bloc have said the election was conducted largely in a fair manner.
Both the US and the UK have expressed serious concern over the situation.
Calling the polling "deeply flawed", US Secretary of State John Kerry said: "In light of substantial electoral irregularities reported by domestic and regional observers, the United States does not believe that the results represent a credible expression of the will of the Zimbabwean people."
Britain, the former colonial ruler of Zimbabwe, has called for a thorough investigation into the election-related allegations. Expressing "grave concern", Foreign Minister William Hague said scores of Zimbabweans were not allowed to vote.
It has been claimed that over 8 million ballot papers were printed, in a country with just over 6.4 million registered voters.
Zimbabwe's Daily News has reported that a batch of ballot papers cast in favour of Tsvangirai have been found in a dustbin.