A session of South Africa's parliament convened for a keynote address by President Jacob Zuma descended into chaos on Thursday as far-left lawmakers brawled with orderlies after interrupting the speech and the main opposition party walked out.

Deputies from the Economic Freedom Fighters (EFF) party fired questions at Zuma for more than an hour, forcing the president - whose authority has been tarnished by a series of scandals - to halt his state-of-the-nation speech.

Zuma had earlier authorised more than 400 soldiers to join the security team outside the building during the speech, an unprecedented move his opponents described as a "militarisation" of parliament.

Speaker Baleka Mbete ordered the EFF contingent ejected after their leader Julius Malema called Zuma "rotten to the core".

Exiting the chamber with his deputies a short time later, opposition Democratic Alliance party leader Mmusi Maimane said the president was unfit to hold office.