Three people were killed in Zimbabwe's capital Harare after troops opened fire on rioting opposition supporters, police say.
The government says the army was deployed in central Harare to help police restore order.
The opposition MDC Alliance condemned the crackdown, saying it was a reminder of the "dark days" of Robert Mugabe's rule.
It alleges that the governing Zanu-PF party has rigged Monday's elections.
Parliamentary results show Zanu-PF heading for a big majority in the first elections since long-serving ruler Mr Mugabe was ousted from power.
The presidential result has yet to be declared. However, the MDC Alliance insists that its presidential candidate, Nelson Chamisa, won Monday's election.
European Union monitors have expressed concern over the length of time it is taking to declare the presidential result.
What are the two sides saying?
President Emmerson Mnangagwa was quoted by state broadcaster ZBC as saying: "We hold the opposition MDC Alliance and its whole leadership responsible for this disturbance of national peace, which was meant to disrupt the electoral process."
He later took to Twitter to urge peace:
As this day that ended in tragedy comes to a close, I implore all political and community leaders to utter these words loudly and clearly to all those who follow them - “Seek Peace and Pursue it!”— President of Zimbabwe (@edmnangagwa) August 1, 2018
Justice Minister Ziyambi Ziyambi said the army had been deployed in Harare to disperse a violent crowd and to restore "peace and tranquillity".
He added: "The presence of the army is not to intimidate people but to ensure that law and order is maintained. They are there to assist the police."
A spokesman for Mr Chamisa condemned the deployment of soldiers and the subsequent loss of life.
"Soldiers are trained to kill during war. Are civilians enemies of the state?" he asked.
"There is no explanation whatsoever for the brutality that we saw today."
Correspondents say the violence was confined to the centre of Harare - an opposition stronghold - while other parts of the country remain calm. Latest reports from the capital suggest the security forces are in control of the streets.
'Chaotic scene of burning tyres'
By Pumza Fihlani, BBC News, Harare
Army vehicles and police trucks rolled into Zimbabwe's main city on Wednesday after the wait for the election results took an ugly turn.
MDC Alliance supporters had been gathering in various parts of Harare 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 previously upbeat mood changed.
Opposition supporters went on the rampage down Harare's busy streets, heading towards an old Zanu-PF office and carrying large stones, sticks and anything else they could grab along the way. The crowd chanted: "We want Chamisa."
They believe the election has been stolen, and are demanding the MDC be announced as the winner.
Riot police using water cannon and tear gas arrived to a chaotic scene of burning tyres and an unrelenting crowd. There were hundreds of them. They jeered and pelted the police vans with stones.
In another part of the city where more opposition protesters had gathered, the army used whips to disperse them.
Today's clashes may not have been on the scale of the "days of old", where intimidation by security police was the order of the day, but it's certainly not the peace many had been praising until now. Something has changed here.
What results have been declared?
The Zimbabwe Electoral Commission (Zec) has announced 140 seats for Zanu-PF so far, and 58 for the MDC Alliance, ZBC state media reported. There are 210 seats in the National Assembly's lower house.
More than five million people were registered to vote, and there was a high turnout of 70%.
ZBC had reported that the electoral commission would announce the presidential results at 12:30 local time (10:30 GMT) on Wednesday, but only parliamentary results were read out.
The BBC's Shingai Nyoka reports that the announcement on the presidential poll was not made because representatives of some of the 23 candidates had failed to turn up to verify the results.
A presidential candidate needs more than 50% of the vote to win outright. Otherwise, a run-off election will be held on 8 September.
What are election observers saying?
The EU mission has criticised the delay in announcing the presidential results. Zec has until Saturday to do so.
It said it had observed several problems, including media bias, voter intimidation and mistrust in the electoral commission, adding that there was an "improved political climate, but un-level playing field and lack of trust".
This is the first time in 16 years that the government has allowed EU and US election monitors into the country.
The African Union mission has said the elections "took place in a very peaceful environment" and "were highly competitive".
It added that it could not confirm opposition parties' complaints of vote-buying, intimidation by the state and bias by traditional leaders.
A preliminary report by the Southern African Development Community (Sadc) observers said the elections were largely peaceful and conducted in accordance with the law.
More on post-Mugabe Zimbabwe
Meet the frontrunners:
Emmerson Mnangagwa, Zanu-PF
- Known as "the crocodile" because of his political shrewdness - his party faction is known as "Lacoste"
- Accused of masterminding attacks on opposition supporters after the 2008 election
- Thought to be 75 years old, he promises to deliver jobs and is seen as open to economic reforms
- Survived several alleged assassination attempts, blamed on supporters of ex-President Mugabe.
Read more: The 'crocodile' who snapped back
Nelson Chamisa, MDC Alliance
- His skull was fractured when beaten up by state security agents in 2007
- Became an MP at 25, a cabinet minister at 31 and could become the youngest president at 40
- A recently qualified pastor, he has been using the hashtag #GodIsInIt for his campaign
- Has promised to rebuild the country's devastated economy, but has been criticised for making extravagant promises - such as the introduction of a high-speed bullet train and bringing the Olympics to Zimbabwe.