The mother of Jean Charles de Menezes wept as she joined a 300-strong protest calling for an end to the deaths of people in state custody.
Maria Otone de Menezes, 63, and her elder son Giovani da Silva, 36, marched to Downing Street alongside people whose loved ones died in police custody, prisons and psychiatric hospitals.
Since 1969 there have been 2,533 such deaths in the UK in which the individuals' names were known and hundreds more unnamed fatalities, according to campaigners.
They said that in the last year alone, 182 people are known to have died in this way, and called for an independent public inquiry into such deaths.
Mr de Menezes, 27, was shot by police after being mistaken for a terrorist at Stockwell Tube station in south London on July 22, 2005.
His cousin Patricia Armani, 34, from Rugby, said: "We're here looking for justice and to support other families that have a similar problem to us. I think it's a sad day, but it's also good because everyone is together looking forward to one thing - justice."
The march marked the tenth year the United Friends and Family Campaign (UFFC) held a remembrance procession in London.
Campaigners gathered in Trafalgar Square to begin a silent march to Downing Street, where a tearful Mrs de Menezes was among a group who laid flowers and pictures of loved ones at the gates while the crowd chanted protest slogans.
The Trades Union Congress (TUC) joined the campaigners' call for an inquiry into deaths in custody.
In a letter to Home Secretary Jacqui Smith, TUC General Secretary Brendan Barber said: "The TUC believes that the state has a duty of care towards the people it takes into custody, and a responsibility to be open and accountable when deaths occur."