Opposition Movement for Democratic Change (MDC) official Tendai Biti said on Tuesday that the party was sure Chamisa had won and that if he was not declared victorious, it would "announce our own results".
With 151 out of 210 parliamentary constituencies counted, Mr. Mnangagwa's ZANU-PF had won 109 seats, while the opposition Movement for Democratic Change gained 41 seats, according to the electoral commission.
Supporters of the opposition party tore down a billboard of incumbent Mr Mnangagwa, and marched to the gates of the electoral commission, which were padlocked by police.
The crowds had been there since the morning but when news came that Zanu-PF had won the majority of seats in parliament and that the presidential results were not ready, the mood turned.
HARARE, Zimbabwe-The party of former President Robert Mugabe took the majority of seats in Zimbabwe's parliamentary elections, officials said, but European and USA observers expressed reservations about the vote and tensions on the streets of the capital mounted.
Tension is rife in Zimbabwe after violence broke out on Wednesday, August 1, following claims by Opposition candidate Nelson Chamisa that the Monday, July 20, election was rigged in favour of President Emmerson Mnangagwa.
The EU's assessment is critical in determining whether Zimbabwe can shed its pariah status as it could help attract investors and trigger an economic revival.
ZEC Chairwoman Priscilla Chigumba said: "It depends with whether those who disagree with something would like to use legal remedies but sometime tomorrow we should be able to advise you what time we can, if any, start announcing the presidential election results".
It is illegal to announce results before the Zimbabwe Electoral Commission.
Angry opposition supporters have gathered outside Zimbabwe's electoral commission, and were met by riot police, as the country awaits the results of Monday's presidential election.
Mnangagwa, a former deputy president who fell out with Mugabe and then took over from him, has said his showing in the election was "extremely positive" while urging people to wait for official results.
"What you see here is real support - not people being brought on buses like ZANU-PF does", he said.
President Emmerson Mnangagwa says the MDC Alliance leadership is responsible for the disturbances that rocked Harare's central business today. The opposition had earlier alleged that its V11 forms shows a win for Nelson Chamisa.
Shots have been fired near protests against the election results in Harare. "We have won this one together".
On Wednesday they announced their findings, noting that while the polls were an improvement on previous ones there had been misuse of state resources and media bias.
Its representative, Angola's Foreign Minister, Manuel Domingos Augusto, called the poll "a political watershed in Zimbabwe's history" that would lead to "consolidation of democracy".
Zimbabwe was once one of Africa's most promising economies but descended into corruption, mismanagement and diplomatic isolation in the latter stages of Mugabe's administration. "No amount of results manipulation will alter your WILL".
European Union observers voiced concern about delays in releasing the results of the presidential contest, a two-horse race between Chamisa and Mnangagwa, head of the ruling ZANU-PF party. Unreasonably inflated figures from so called ZANU-PF strongholds are worrying against the backdrop that in 21% of polling stations mostly in those areas results were not posted as required by the law.