Born in 1954 in the coastal city of Balteem in the Delta governorate of Kafr El-Sheikh to a working-class family, Hamdeen Sabahi was the youngest of 11 children.
He studied mass communication at Cairo University.
A politically-active student, he was elected head of the Cairo University Student Union in 1975 and served as deputy chair of the General Federation of Students from 1975 to 1977.
He established the political Nasserist Thought Club to mobilise on Cairo University’s campus in defence of the principles of Gamal Abdel-Nasser's 1952 revolution in a context of later rapprochement efforts with the West by then-president Anwar El-Sadat.
As well as being a political activist, Sabahi worked as a journalist.
In 1977, following the January popular uprising against Sadat over skyrocketing food prices, Sabahi publicly confronted the president in a televised meeting in which he spoke on behalf of the Cairo University Student Union. As a result, Sabahi was prohibited from working as a journalist in the state media sector for several years.
Sabahi’s membership in the Arab Democratic Nasserist Party was suspended in 1994 due to internal conflicts between the party's young cadres – including Sabahi – and the party's old guard of personalities who had been close to late president Nasser.
In 1996 Sabahi founded the Arab Nasserist Karama Party and was elected to the People's Assembly in 2000 and 2005.
Sabahi was jailed several times over the course of his political career. One of these arrests was under Sadat in 1981, when he was one of around 1,500 politicians, intellectuals and activists arrested in what was a major crackdown on opposition from across the political spectrum.
Sabahi was again arrested in 1997, under the regime of former president Hosni Mubarak, on charges of inciting agricultural workers to protest against new legislation that strengthened the hand of landowners against poor tenant farmers.
In 2003, as a sitting MP, his parliamentary immunity was lifted and he was subsequently jailed for his involvement in organising demonstrations against the US-led war on Iraq that year.
Sabahi was also among the co-founders of the pro-democracy Kefaya (Enough) movement, which emerged in 2004. The movement played a significant role in building opposition to the unpopular regime of Mubarak, especially his attempts to groom his son, Gamal, to succeed him as president.
In 2010, he was one of the co-founders of the National Assembly for Change (NAC), which sought constitutional reform and social justice. Other prominent NAC members included former director of the International Atomic Energy Agency Mohamed ElBaradei, writer and activist Alaa El-Aswani and human rights activist George Ishaq.
He participated since the 1970s in establishing solidarity campaigns and committees with both Lebanese and Palestinian groups opposed to the normalisation of relations with Israel.