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.
Edgar Lungu is running for re-election in August's polls.
At least six people have been killed and mutilated in Lusaka in recent weeks.
Zambia's kwacha has gone from the world's third-worst to its best-performing currency.
Erick Chanda suggested President Lungu spent taxpayers' money on holiday in 2015.