Video footage emerged on Friday morning and it shows an EFF MP slapping a man after Thursday evening's State of the Nation Address.
It is believed that the man on the receiving end of the slap is a police officer from the Presidential Protection Service.
The video apparently shows EFF MP Marshall Dlamini and other EFF MPs, including party leader Julius Malema, expressing unhappiness about not being allowed into the lobby in front of the National Assembly as fast as they wished.
Malema can be seen wagging his finger in the man's direction before Dlamini slaps him.
A woman then pushes Dlamini away, before another man gets in between Malema and the person who was slapped.
ACDP MP Steve Swart was a witness to the incident, and can be seen in the video toward the back of the crowd.
Swart told News24 they had been waiting for the presidential party, who were taking pictures in the lobby, to leave. There were four policemen in uniform and another policeman in plain clothes who asked him and others to wait.
"We were waiting there and then the EFF came through and said: 'Excuse me, we have work to do,' and pushed through the police. And that's when the altercation took place."
Swart told News24 that he couldn't see which MP was involved, but definitely heard "a klap [smack]" from his vantage point.
"The policeman actually staggered right back, it was a hard hit because the blood started to flow. I don't know if anyone else [was hit] but it was shocking actually."
EFF spokesperson Mbuyiseni Ndlozi could not be reached for comment on Friday morning.
News24 is also waiting for a response from Parliament.
The story will be updated once further information is known.
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