Emmerson Mnangagwa declared winner in disputed Zimbabwe election
Police removed opposition officials from the electoral commission stage when they rejected the results.
The chairman of Mr Chamisa's MDC Alliance said the count could not be verified.
By narrowly winning more than 50% of the vote, Mnangagwa avoids a run-off election against Chamisa.
Mnangagwa took to Twitter to say he was "humbled", and called the result "a new beginning".
Mnangagwa, from the ruling Zanu-PF party, took over as president last November from long-serving leader Robert Mugabe.Thank you Zimbabwe!— President of Zimbabwe (@edmnangagwa) August 2, 2018
I am humbled to be elected President of the Second Republic of Zimbabwe.
Though we may have been divided at the polls, we are united in our dreams.
This is a new beginning. Let us join hands, in peace, unity & love, & together build a new Zimbabwe for all! pic.twitter.com/FbdrixAktR
Chamisa has insisted he is the winner of the presidential poll, telling reporters earlier on Thursday that Zanu-PF was "trying to bastardise the result", something "we will not allow".
Six people died after opposition protests in Harare on Wednesday over alleged vote-rigging.
The elections were the first since Mr Mugabe, 94, was ousted and were intended to set Zimbabwe on a new path following years of repressive rule.