"No candidate has received a majority of votes counted. A second election will be held at a date to be announced," the commission said in a statement.
Mugabe, 84, won 43.2 percent of votes in the March 29 election, the Electoral Commission said in announcing the long-delayed results. Independent Simba Makoni, a ruling party defector, took 8.3 percent.
The opposition Movement for Democratic Change called the announcement of the long-delayed result "scandalous daylight robbery," reported Reuters. The party says Tsvangirai won more than 50 percent at the March 29 election and Mugabe's 28-year rule is over.
"[T]here's no outright winner, pointing to a runoff," Bright Matonga, a spokesman for Mugabe's Zanu-PF told the BBC.
"The laws of Zimbabwe and the constitution clearly states that for one to be an outright winner, they have to achieve 50 [percent] plus one. If no one achieves that, then there's going to be a runoff so we are following our constitution, not people's wishes," Matonga said, adding the constitution required a second round be held no sooner than 21 days from the announcement of the results.
"[I]t would be rather hard to see how there could be a fair runoff election," State Department deputy spokesman Tom Casey said Thursday at a news conference. "In fact, I think it would almost be impossible to hold one, given the current campaign of state-orchestrated violence and intimidation against the political opposition, in particular, and against just much of the general citizenry."
Tsvangirai has said previously that he will not participate in any runoff.
Even before the announcement was made Friday, the MDC challenged the process, saying Thursday there were 120,000 unaccounted votes that could give them outright victory.
Earlier Friday, Tsvangirai's spokesman George Sibotshiwe told the Associated Press the MDC anticipated needing another three or four days to examine the verification of the results.
Tsvangirai has been out of the country since shortly after the vote, trying to keep up international pressure on Mugabe.
Mugabe's party lost power of the parliament for the first time in March, a sign of frustration over the economic crisis in Zimbabwe, where inflation is at 165,000 percent.