The veteran leader scored another five years in office, extending his 33-year rule with a landslide 61 percent of the vote, against Tsvangirai's 34 percent. (Agencies)
In Parliament, his ZANU-PF party scored a super majority which allows it to make changes to the country's constitution.
The result came as a massive blow to longtime opponent Tsvangirai, who said his Movement for Democratic Change "totally" rejected Wednesday's vote and would boycott the incoming government.
The election ends an uneasy power-sharing government with Mugabe installed in 2009 after another disputed vote. Prime Minister Tsvangirai swiftly ruled out joining Mugabe's government again.
"We will not join government. We will go to court," he insisted, after his party held emergency talks to plot its next move," he said.
The MDC rejected the results but has stopped short of calling for mass protests, as growing tensions spark fears of a repeat of the bloody violence that marked the aftermath of the 2008 election.
Amid observer concerns over the electoral roll and high numbers of voters being turned away, the poll's credibility was hit by the resignation of one of the nine official electoral commissioners.
In a letter that was sent to Mugabe and Tsvangirai, dated the day of the polls, Mkhululi Nyathi said he had quit over "the manner" in which the polls "were proclaimed and conducted".
"While throughout the whole process I retained some measure of hope that the integrity of the whole process could be salvaged along the way, this was not to be, hence my considered decision to resign," he said.
The MDC now has until Wednesday to present evidence of fraud to the high court, but finding a smoking gun may prove difficult.
UN chief Ban Ki-moon has urged both political rivals to send "clear messages of calm" to their supporters.
Harare barber Right Chirombe said there was no basis to the claims that the poll was rigged.
The influential 15-member Southern African Development Community has also implored "all Zimbabweans to exercise restraint, patience and calm". The bloc stopped short of declaring the vote "fair" but said it was "free and peaceful".
The veteran leader scored another five years in office, extending his 33-year rule with a landslide 61 percent of the vote, against Tsvangirai's 34 percent.