As the tension between police and protesters ramped up on Wednesday, the authorities deployed the military to the streets, who used live rounds to quell the unrest. Some journalists were also attacked.
Automatic gunfire has been heard all afternoon as troops attempt to disperse protestors.
Zimbabwe's electoral commission had said it would start announcing results for the presidential race on Wednesday but that was then pushed back at least 24 hours. At least three protesters have been shot and killed. There were pools and trails of blood on the streets.
But the mission welcomed an "improved political climate" and largely peaceful election, a break from past violence under longtime autocratic leader Robert Mugabe. "People were happy on Monday. We are not going to accept defeat".
What appears to be a mother and children flee violence in Zimbabwe's capital.
"This is due to the fact that the degree of disturbances of law and order have reached a magnitude where by the regular police officers and the reserve members have been unable to cope", said Senior Assistant Commissioner Charamba.
While the presidential contest involved 23 candidates, the main race was between Chamisa, a 40-year-old lawyer and pastor, and 75-year-old Emmerson Mnangagwa, who helped engineer Mugabe's downfall in November.
He said: "The force used today was clearly disproportionate and unjustifiable".
"Are we in war? Are civilians enemies of the state?" he asked.
UN Secretary General Antonio Guterres urged Zimbabwe's politicians to exercise restraint, while United Kingdom foreign office minister Harriett Baldwin said she was "deeply concerned" by the violence.More news: Judge blocks release of 3-D printed gun blueprints
"You can't rig our election.this is a military government", the crowd shouted.
On his official Twitter feed, Chamisa accused the ZEC of releasing the parliamentary results first to prepare Zimbabweans for a Mnangagwa victory.
The opposition alleged irregularities, saying results were not posted outside one-fifth of polling stations as required by law.
Opposition leader Chamisa accused the ZANU-PF of trying to steal the election after official figures gave it a two-thirds majority in parliament.
The commission said it would release vote totals "sometime" on Thursday, even though it said most of the results "are here with us".
"We call on the political leaders and the population as a whole to exercise restraint and reject any form of violence while awaiting resolution of the disputes and announcement of the election results".
Anti-riot police backed by water cannon trucks monitored the demonstrators, while MDC supporters also protested outside the conference center where election results are being announced. Both the European Union and U.S. missions urged the release of the presidential results as soon as possible.
Many Zimbabweans looked to the vote as a launching pad for a return to the kind of worldwide acceptance and relative prosperity that the country enjoyed in the first part of the rule of Mugabe, who took office after independence from white minority rule in 1980. On Wednesday, they came to enforce a crackdown. "This time we will not allow it, we will fight", said one protester who wore a red MDC beret in central Harare.
Jack Mufambi, a resident walking near the ruling party headquarters, said he was scared to go outdoors on Thursday.