Protestors pushed past police invading the courtyard area of the building, after Councillors passed an Alliance amendment supporting the flying of the Union flag on designated days.
Councillor Laura McNamee said: “The DUP and UUP have encouraged people to come out and protest tonight, yet as soon as they lose control of the situation they look for someone else to blame as they attempt to wash their hands of any involvement.
“They cannot ignore their involvement in the Council being brought to a complete lock down tonight. Through their continued campaign of harassment against the Alliance Party they have stirred up Unionist fears for their own political gain.
“A number of Alliance Councillors and staff members – myself included – have been subjected to serious threats in the past few weeks. This is completely unacceptable and unnecessary and due to the actions by many in the chamber tonight.
“We recognise anyone’s right to a peaceful protest, however what has been taking place outside Belfast City Hall tonight has been neither peaceful or respectful. This was proven when a number of protestors pushed past police and invaded the City Hall.”