Mugabe, 89, who has run the country since he helped end white rule in 1980, trounced his long-standing political rival, Morgan Tsvangirai, in Wednesday's election, Zimbabwean election officials said.
By the time he completes his new tenure, he will have ruled the former British colony in southern Africa for 38 years.
Official results showed Mugabe won 61 per cent of the presidential vote and his party got a super majority in parliament that will allow it to change the constitution. He routed Tsvangirai who trailed heavily with 34 per cent.
The reaction in today press was divisive with state-controlled newspaper The Herald proclaiming "President Mugabe romps to victory", while the independent Daily News said "It's a crisis".
More From This Section
US Secretary of State John Kerry meanwhile described the election as "deeply flawed" and said the US "does not believe that the results ...Today represent a credible expression of the will of the Zimbabwean people."
British Foreign Secretary William Hague added his own "grave concerns" over the conduct of the vote in the former colony.
"Given our doubts about the results, Australia calls for a re-run of the elections based on a verified and agreed voters roll," Foreign Minister Bob Carr said in a statement.
Tsvangirai vowed to challenge the result in court and said his Movement for Democratic Change (MDC) would boycott government institutions.
"We will not join government," he said. "We will go to court."
"The fraudulent and stolen election has plunged Zimbabwe into a constitutional, political and economic crisis."
With gentler assessments from African observers who nonetheless noted flaws, President Jacob Zuma of powerful neighbour South Africa offered his "profound congratulations" to Mugabe today.