Mr Mugabe, who won 61% of the vote, was re-elected for another five-year term on Saturday, the Zimbabwe Election Commission said.
His ZANU-PF party won 158 of the 210 parliament seats, giving it a two-thirds majority in the legislature that enables it to make amendments to the new constitution and existing laws.
“Mugabe, Robert Gabriel, of ZANU-PF party is therefore duly elected president of the Republic of Zimbabwe with effect of today,” commission head Rita Makarau told a news conference.
Opposition party Movement for Democratic Change (MDC) said on Saturday it would not participate in any government resulting from what Mr Tsvangirai called a “fraudulent” election.
Mr Tsvangirai, denounced the July 31 election as a “huge farce“, alleging massive rigging by ZANU-PF. Zimbabwe’s largest domestic observer group has also called the elections “seriously compromised.”
A Zimbabwean election commissioner also resigned on Saturday, citing doubts about the integrity of results showing a big win for president Mugabe’s ZANU-PF party but dismissed as a fraud-riddled farce by his main challenger.
Mkhululi Nyathi said he quit the nine-member Zimbabwe Electoral Commission (ZEC) over the way it managed the presidential and parliamentary vote held on Wednesday. His resignation is likely to add to the dispute over the election both inside and outside Zimbabwe.
“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,” Nyathi said in a resignation letter on Saturday.
Mr Tsvangirai’s Movement for Democratic Change (MDC) said on Friday it could take to the streets to challenge ZANU-PF’s claim of a landslide victory, made less than 24 hours after the polls had closed on Wednesday.
The vote passed off peacefully and received broad approval from African observers.
Africa’s oldest leader, president Mugabe has governed the former British colony, then known as Rhodesia, since independence in 1980.