The Economic Freedom Fighters (EFF), led by Julius Malema, repeatedly interrupted Jacob Zuma, demanding answers over a spending scandal.
The speaker of parliament then ordered their removal, prompting scuffles.
The largest opposition party, the Democratic Alliance, walked out in protest at their expulsion.
“You can’t send police into parliament,” said Democratic Alliance parliamentary leader Mmusi Maimane.
The EFF used President Jacob Zuma’s annual State of the Nation speech to question him about a state-funded, multi-million dollar upgrade to his private residence.
They were evicted by large numbers of security guards. Among those thrown out was Julius Malema.
Julius Malema defended his lawmakers’ actions, calling it a legitimate attempt to defend democracy, adding that seven of his party had been injured and would be pressing charges.
It is not clear whether the guards were police or parliamentary officials: an important distinction, according to one lawmaker, who said the use of police would be a way of intimidating the opposition.
Julius Malema and Jacob Zuma were once close allies but the pair fell out.
The EFF has shaken up South African politics with a series of populist proposals to redistribute wealth.
EFF lawmakers accuse Jacob Zuma of benefitting unduly from taxpayer-funded upgrades to his private residence in the village of Nkandla and they wanted the president to answer questions about this before making his state of the nation speech.
In 2014, an independent inquiry found the president had “unduly benefited” from the expensive upgrades, which included a pool and a cattle enclosure and cost about $23 million.
President Jacob Zuma has denied any wrongdoing.
Despite criticism of the Nkandla upgrade and South Africa’s stagnant economy, Jacob Zuma’s African National Congress was overwhelmingly re-elected last year.
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