A memorial honouring a British human rights campaigner who was among those killed in the July 7 London bombings three years ago was unveiled on Saturday in a West London park.

Giles Hart, 55, who was a BT engineer and lived in Hornchurch, Essex, was killed instantly when the No 30 bus on which he was travelling to work at the Angel, in North London, was blown up in Tavistock Square.

In the 1980s Mr Hart had been a key figure among international supporters of the struggle of the Polish trade union Solidarnosc for its rights during Poland's days of communist repression. In the 1980s, the Polish Solidarity Campaign (PSC), a non-party group, was the main British pro-Solidarnosc organisation, and Mr Hart served as its chairman and its treasurer while Poland was under martial law.

On Saturday the chairman of Solidarnosc, Janusz Sniadek, who had travelled from Gdansk for the ceremony in Ravenscourt Park, Hammersmith, paid grateful tribute to his campaigning efforts. Mr Sniadek told a crowd of about 80, including MPs, councillors and Mr Hart's friends and family, that Polish society owed much to those abroad such as Mr Hart, a man "great in spirit", for their contributions to the peaceful struggle that led to the collapse of totalitarianism.

The memorial stone - a 1.3 metre polished slab of Silesian granite imported directly from the Strzegom quarries in Poland - was unveiled by the Polish Ambassador, Barbara Tuge-Erecinska, and the Mayor of Hammersmith & Fulham, Councillor Andrew Johnson, who told the crowd that it was a fitting epitaph to "a truly great man". Poles have settled in the area for generations, and the PSC, the British organisation that Mr Hart had led, was based there.

The inscription on the memorial - in English and Polish - describes Mr Hart as "a lifelong campaigner for freedom and human rights, honoured by Solidarnosc as one who supported Poland in her hour of need". It also bears a quotation from Mahatma Gandhi, "Be the change you want to see in this world", which was chosen to express Mr Hart's commitment to working for a better world.

In a personal tribute, Karen Blick, of the Giles Hart Solidarity Memorial Committee - which organised the event - told the gathering of her friend Mr Hart''s "love of freedom and hatred of oppression". She said: "We remember him for his transparent honesty and his tenacity." A trade unionist himself, Mr Hart thought that everyone all over the world had a right to belong to a free trade union, Mrs Blick said.

The ceremony was also attended by Greg Hands, MP for Hammersmith & Fulham, Andrew Slaughter, MP for neighbouring Ealing, Acton & Shepherd's Bush, and Stephen Greenhalgh, leader of Hammersmith & Fulham Council, and the TUC was represented by Sean Bamford, of its European Union and International Relations Department.

A minute's silence was observed to commemorate all those killed in the London bombings, and flowers were placed at the memorial stone by another of Mr Hart's friends, Anna Lubelska.

After the ceremony, the Polish Social and Cultural Association hosted a reception at its Hammersmith centre, where the Polish Ambassador said that Mr Hart "will always be remembered in Poland as a great friend of my country". His work for democracy had been recognised in July 2005 when he was posthumously awarded the Knights Cross of the Order of Merit of the Republic of Poland by the President of Poland. Mrs Tuge-Erecinska expressed "on my own behalf of and on behalf of the Polish people, deep gratitude and appreciation for the wonderful work that was done by Mr Hart ".

Last week, the TUC General Secretary, Brendan Barber, said: "Giles Hart was a committed union member who spent years campaigning for trade union rights and basic human rights freedoms. This memorial is a fitting tribute to his life, cut tragically short by the terrorist attacks of 7 July 2005."

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