The decision was taken to ensure Mmusi Maimane did not put "undue pressure" on the judiciary, a Zambian diplomat said.

Mr Maimane said his expulsion showed Zambia was becoming a dictatorship.

He had planned to show solidarity with the southern African state's main opposition leader, Hakainde Hichilema, during his court appearance on Friday.

Mr Hichilema was arrested in a raid by armed policemen on his home in Zambia's capital, Lusaka, in April.

He was charged with treason after his convoy allegedly refused to make way for President Edgar Lungu's convoy, while both were travelling to a ceremony in western Zambia.

The magistrate's court ruled that Mr Hichilema's lawyers could seek "a judicial review" of the charges after his lawyers argued for the case to be thrown out.

Mr Maimane condemned the "trumped up" charges against Mr Hichilema, his Democratic Alliance (DA) party said in a statement.

He was "aggressively confronted" by police when his plane landed, and he was "forcefully" prevented from entering Zambia, his Democratic Alliance (DA) party said in a statement.

The Zambian government's behaviour was "shameful" and South Africa should lodge a diplomatic protest with it, the party added.

Maimane applauded: Pumza Fihlani, BBC News, Johannesburg

"All I simply wanted was to stand side-by-side with someone who has been fighting for the rule of law, HH [Hakainde Hichilema, a fellow African," Mr Maimane said, just hours after he was unceremoniously booted out of Zambia.

While some have questioned why he would involve himself in the issues of a sovereign state, many have applauded him for trying to highlight the Zambian opposition leader's plight.

Mr Maimane says that he was compelled to intervene because of South Africa's own fight against oppression.

He has been calling for support on social media using the thread #iCantBeSilent.

Authorities in South Africa are keen to avoid a diplomatic row with Zambia, but say they will look into the matter.

Zambia's top diplomat in South Africa, Emmanuel Mwamba, said Mr Maimane's presence in court would have undermined "the sanctity, integrity and independence of the judiciary".

"It was imperative that the due process of the law was respected without undue pressure or interference," he told journalists.

Treason is a non-bailable offence in Zambia, with a minimum jail term of 15 years and a maximum sentence of death.

Last year, Mr Lungu narrowly beat Mr Hichilema in disputed presidential elections.