Chamisa, who heads the Movement for Democratic Change (MDC), narrowly lost to Emmerson Mnangagwa in Zimbabwe's first election since the ousting of long-time ruler Robert Mugabe last year.
Chamisa's legal bid to have the result overturned due to alleged electoral fraud and irregularities was rejected by the constitutional court.
"He will be recognised as the legitimate president of Zimbabwe by his party and the people of Zimbabwe following resolutions of the national council," MDC spokesman Nkululeko Sibanda told AFP.
Sibanda said Chamisa had been denied victory by "cheating and chicanery" and that the ceremony would be held on Saturday, when the party celebrates its 19th anniversary.
"Resolutions will be passed to recognise his victory and state it publicly," he said.
Mnangagwa, of the ruling ZANU-PF party, won the election with 50.8 percent of the vote -- just enough to meet the 50 percent threshold needed to avoid a run-off against Chamisa, who scored 44.3 percent.
Mnangagwa had pledged free and fair elections to restore relations with the west, but the vote was marred by the fatal shooting of at least six people when soldiers were deployed to quell opposition protests.
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