Mmusi Maimane traveled to Zambia on Thursday to attend the trial of the country's embattled opposition leader, but says he was "assaulted" and turned back upon arrival.
The country's president wants to stay in power beyond his limit and has put his rival on trial for treason.
Lungu took 50.35 percent of the vote, but his main rival is challenging the result in court.
Edgar Lungu is close to remaining as Zambia's president.
The ministers stayed in power illegally following the dissolution of parliament in May.