Tsvangirai arrived at Harare airport aboard a regular South African Airways flight around 11:30 a.m. British time (1030 GMT) after cancelling his homecoming a week ago after his Movement for Democratic Change (MDC) said it had learnt he was the target of a military intelligence assassination plot.
The government dismissed the plot as a propaganda stunt.
Tsvangirai told a news conference that Mugabe and the ruling ZANU-PF party had launched a concerted campaign against the MDC, which has seen 42 people killed and tens of thousands displaced.
"ZANU-PF wants to decimate MDC structures," Tsvangirai said, adding that many opposition officials were in hiding.
He said he was confident of victory, although conditions are not conducive for a free and fair election and ZANU-PF was trying to destroy his MDC before the run-off.
"The conditions on the ground for a run-off are not perfect, and will never be perfect. But we are saying with the support of SADC (Southern African Development Community), putting in election observers and peacekeepers, we can instil confidence in the people of Zimbabwe".
The MDC chief was cheered by party officials at the news conference when he vowed to knock out Mugabe in the second round, saying he was drawing fresh inspiration from victims of political violence:
"I was in the hospital today, people with scars, wounds, all saying: 'President, we will finish him off, don't let us down'."
Tsvangirai has been travelling abroad since April 8 on a diplomatic drive to pressure Mugabe to surrender power following a March 29 presidential poll, which he says he won outright.
But Zimbabwe's electoral commission says he did not get enough votes for a straight victory and must face Mugabe in a June 27 run-off.
