A large protest was held in Bristol UK repression of protests and the plans to restrict them more. The protesters attacked cops violently and injured some severely.

Bristol is a rather leftish city which elects a Green representative to Parliament, so it makes sense that a big protest would happen there, and that local officials would support the purpose.

I expect the violence did harm to the cause of defending the right to protest, because it will allow the cops (who did not, it seems, act like thugs on this occasion) a justification for the claim they need more powers — which would enable the to repress nonviolent protests more in the future,

It is not a valid justification, but they will make use of it anyway.

The left needs to adopt disciplined nonviolence, such as it employed effectively in the 60s and 70s in the US, and reject "black bloc" tactics. As long as it does not, it is vulnerable both to hot-headded supporters and to right-wing provocateurs.