Ian Tomlinson's widow and son joined a crowd of about 50 supporters in central London, where a minute's silence was held.
Mr Tomlinson, 47, was walking home from work through the City of London on 1 April last year when he was struck with a baton and pushed to the ground during clashes between G20 protesters and police.
Politicians, campaigners, lawyers and academics have sent a letter to the Director of Public Prosecutions criticising the "intolerable" delays surrounding the investigation into his death.
His family is still awaiting a reply from the Crown Prosecution Service (CPS) over whether it plans to charge anyone.
Mr Tomlinson's widow Julia said yesterday: "We want justice. It's been a very difficult year. We want to move on. Until this is all sorted out, our lives are on hold."
She said the investigation into the death was taking "far too long" and she blamed the CPS for the delays.
The dead man's son Paul King said: "We just want justice, really. We just want to get our life back. It's been a long wait."