Voters turned out in huge numbers to cast their ballots in the first election since Robert Mugabe was ousted as president.
In response to the comments, Mr Mnangagwa accused his former boss of making a deal with the opposition.
"I have no doubt that by end of day today we should be very clear as to an emphatic voice for change", Chamisa said.
Slow and rambling, the 94-year-old Mugabe spoke to reporters on Sunday with bitterness about his dramatic removal in November under military pressure and amid a ruling party feud.
"I hope the choice of voting tomorrow will throw, thrust away the military government and bring us back to constitutionality", said a frail looking Mugabe, in a rambling off the cuff speech that lasted nearly an hour.
"I can not vote for those who tormented me", he said.
Mugabe, one of the last "Big Men" of African politics, still looms large over Zimbabwean society and his endorsement of the opposition may yet influence the first vote without his name on the ballot paper since independence from Britain in 1980.
The victor faces the task of putting Zimbabwe back on track after 37 years under Mugabe, tainted by corruption, mismanagement and diplomatic isolation that caused a crisis in a country that once had one of Africa's most promising economies.
Who make up most of the voters?
In Harare, 32-year-old finance graduate Tinashe Dongo said he wanted "change" following Monday's vote.
Aaron Ramsey gives Arsenal transfer update over new contract
Unai Emery has challenged the rest of Arsenal's young players to follow the example set by Emile Smith Rowe. I'm very happy with his work on the pitch but it's not only today, it's every day.
Early polling station results seen by the Guardian showed Chamisa winning by wide margins in urban strongholds and in parts of the strongly anti-government south-west.
Mr Mugabe had a bad relationship with Western powers, accusing them of undermining Zimbabwe's sovereignty and trying to topple hm.
"Once re-elected on Monday with a real mandate for change and a full five-year term, I guarantee you it's "Go and Go" in our country".
"For the first time ever we have now a long list of aspirants to power", Mr Mugabe said.
But campaigning has been relatively unrestricted and peaceful.
The presidential election is expected to be a tight contest between the incumbent Emmerson Mnangagwa and his main rival Nelson Chamisa. Both vice-presidents were slightly injured in the blast, as were several other top officials in the ruling Zanu-PF party. But there is consensus that the process has been better than before.
Mnangagwa invited global observers - including the previously-banned European Union mission - to witness voting.
If Chamisa disputes the result or no one candidate gets more than 50 percent and there is a runoff, there are fears of street protests and possible violence. Tsvangirai won 47.9% of that vote to Mugabe's 43.3%, which necessitated a run-off under constitutional law.
The elections have been opened up to worldwide observers, though the opposition has voiced concerns that the voter roll has irregularities and claimed that the army has been deployed to rural areas in an attempt to intimidate people to vote for the ruling ZANU-PF.
"Although an outbreak of widespread violence as in 2008 is not expected. incidents of localized violence are increasing in frequency and intensity".