MyCiTi employees have been striking for weeks over certain labour demands.
Tweets and video on social media showed a group of protesters on the ground floor of the civic centre on Wednesday, surrounded by police.
Teargas and stun grenades were also fired in the area around the civic centre as at 11:20.
One of the protest leaders, Johannes Gordon, claimed that they entered the building because the city’s metro police said they would be assisted in speaking with new mayor Dan Plato.
He said metro police and South African Police Service members surrounded them on the ground floor. All exits were blocked.
City media manager Luthando Tyhalibongo said they were attending to the situation, and would update the media on the developments shortly. DM