Police were out in force, manning roadblocks on the route from the airport to Harare, the capital, as Tsvangirai left the terminal.
The leader of the Movement for Democratic Change (MDC) had called off a return flight a week before after his party said he was the target of a military assassination plot.
He has survived at least three assassination attempts.
"I feel quite safe," Tsvangirai had told the Associated Press on his way to the airport in Johannesburg, South Africa.
In an apparent reference to the alleged assassination attempt, Tsvangirai told reporters in Harare that "measures have been taken to reduce the risk, but removing the risk doesn't mean the risk will not exist in future".
He said he was optimistic about winning the run-off, adding: "If Mugabe thinks he has beaten people into submission, he will be in for a rude shock on June 27th. I went to hospital to visit victims of the violence and they said 'we'll finish him [Mugabe] off - don't let us down.'"
Supa Mandiwanzira, Al Jazeera's correspondent in Harare, said conditions in Zimbabwe can't allow a free and fair elections but that Tsvangirai was "confident of victory".
"He has asked the Southern African Development Community (SADC) to send election observers by June 1 and that if they don't, they shouldn't bother," he said.
The former trade union leader won the March 29 first round of voting against Mugabe and one other candidate but did not secure enough votes to avoid a second round against Mugabe.
Human rights groups say opposition supporters have been targeted in a campaign of violence aimed at ensuring the 84-year-old Mugabe - who has been in power since the states independence from Britain in 1980 - wins the second round.
Among the assassination attempts 56-year-old Tsvangirai has survived was one in 1997 by unidentified assailants who tried to throw him from a 10th-floor office window.
He was hospitalised after a brutal assault by police at a prayer rally last year and images seen around the world of his bruised and swollen face have come to symbolise the plight of dissenters in Zimbabwe.
Tsvangirai left Zimbabwe on April 8 on an international tour to rally support for democracy in Zimbabwe.
"I'm sure that we have managed to ensure an African consensus about the crisis in Zimbabwe," he said, adding he now wanted to rally his supporters in Zimbabwe.
Tsvangirai's party says more than 30 of its supporters and activists have been killed since the first round of voting and that attacks are increasingly targeting its top leaders.