Updated

The leader of South Africa's main opposition party on Friday sharply criticized Zambian officials who denied him entry to their country as he tried to show support for a jailed opposition leader.

Mmusi Maimane of the Democratic Alliance said officials boarded the plane on arrival in Zambia and "started to assault" him, temporarily confiscating his mobile phone and iPad and preventing him from leaving the aircraft. He returned to South Africa on Thursday.

The treason case against Zambian opposition leader Hakainde Hichilema is politically motivated, Maimane said. Authorities have accused Hichilema of blocking Zambian President Edgar Lungu's motorcade with his own convoy last month.

Hichilema, who leads the United Party for National Development, appeared in court on Friday. The next hearing is scheduled for June 12.

Amnesty International criticized the case against Hichilema and five other party members, calling their arrests "part of a cynical ploy to silence all political opposition in Zambia."

Speaking outside the Zambian embassy in South Africa's capital, Pretoria, Maimane called his treatment by Zambian authorities unlawful and he urged the South African government to formally ask Zambia why he had not been allowed entry.

Emmanuel Mwamba, Zambia's ambassador in South Africa, said his country is entitled to decide who can enter and that Maimane had been advised to postpone his visit to support Hichilema, the African News Agency reported.

"His visit was to go and visit his colleague in solidarity and we found nothing wrong with that," Mwamba said. "But we advised him that there were concerns over his statements and utterances, especially regarding the sanctity of our courts."

The treason case comes after Hichilema challenged Lungu in an unusually tense election last year that saw political violence. For years, Zambia had experienced peaceful power transitions that the United States had praised as a "model for Africa."