Despite O'Neill's initiatives, many Catholics were impatient with the pace of reform and remained unconvinced of the prime minister's sincerity. The result was the founding of the Northern Ireland Civil Rights Association (Nicra) in 1967.
Nicra did not challenge partition - probably in an attempt to draw as much cross-community support as possible - although the membership remained predominantly Catholic. Instead, it called for the end to seven 'injustices', ranging from council house allocations to the 'weighted' voting system.
Initially peaceful civil rights marches descended into violence in October 1968 when marchers in Derry defied the Royal Ulster Constabulary and were dispersed with heavy-handed tactics.
The British government summoned O'Neill to London to explain the situation. Pressure was brought to bear, and shortly afterwards a package of reforms was announced by the Northern Ireland government, including the fairer allocation of council houses and an ombudsman for complaints.
'The reforms failed to deliver one-man-one-vote and the repeal of the repressive Special Powers Act.'
But the reforms failed to deliver fully on Nicra's programme, including one-man-one-vote and the repeal of the repressive Special Powers Act.
After a brief cessation, the civil rights marches continued, organised at first by a group called People's Democracy and later by Nicra. Once again, the RUC response was heavy-handed and would only serve to inflame the Catholic community further.
Increasingly embattled by dissent in the UUP, O'Neill gambled everything on a general election - which he dubbed the 'crossroads election' - to try and win a mandate for change from the public.
The gamble failed amid poor electoral support and desertions from O'Neill's camp. Nonetheless, he hung on grimly for another two months before resigning in April 1969.