On his website, Mr Abse, who was born in Cardiff in 1917 and lived in Chiswick, west London, was described as being politically active from an early age.
According to his website: "In 1944, while serving with the RAF in the Middle East, he was arrested and detained for political activities following upon his contribution to the setting-up of a Forces Mock Parliament in Cairo - an episode that precipitated a parliamentary debate in Westminster."
On returning to Britain he became a solicitor and founded his practice Leo Abse and Cohen, based in south Wales, in 1951.
It now employs around 150 people and has offices in Swansea and Newport as well as Cardiff, where it is one of the city's biggest solicitor firms.
Social reformer Leo Abse dies
He became an an MP in a by-election in Pontypool in 1958 and remained until his retirement at 70 in 1987.
He remained in parliament for 29 years, holding the seat for Pontypool from 1958 to 1983 and then Torfaen from 1983 - 1987.
Having inspired nine Private Member's Acts, he was noted as one of Britain's top social reformers and was well known for his flamboyant style.
In a statement, the firm said: "The partners and everyone at Leo Abse and Cohen are deeply saddened by the news.
"Leo Abse was both the founding partner and a former senior partner of the firm.
"His contribution was immense, not only to us as a firm but also to the wider legal world in Wales and beyond, both as an accomplished lawyer, and in his role as a respected and groundbreaking law maker.
Mr Abse wrote political books based on his interest in psychoanalysis
"He laid the foundations for the firm almost 60 years ago and his core belief in 'access to justice for all' continues to be instilled in the philosophy of our work today."
Torfaen Council leader Bob Wellington said Mr Abse was a "great politician" who "was never scared to start debates on subjects which other MPs avoided".
"Leo Abse was small in stature but he was an intellectual giant," he said.
"He was a great parliamentarian who championed the rights of his constituents for nearly 30 years with a passion and tenacity that left its mark on the borough and right across the UK through significant legal reform.
"Leo spoke regularly on national issues in Parliament but never forgot his main priority, the people of Torfaen.
"He will be sorely missed and can rightly be termed Britain's top reformer of the century."
Mr Abse was the son of Rudolf Abse, a Jewish solicitor and cinema owner who lived in Cardiff.
His younger brother is Dannie Abse, a renowned poet.
He attended Howard Gardens High School in Cardiff followed by the London School of Economics where he studied law.
In 1955 he married Marjorie Davies, an artist with a national reputation for her fabrique collage work and head of the pedagogic department at Cardiff College of Art.
They had two children: Tobias, a lecturer in European History, and Bathsheba, one-time curator of the Keats Museum in Rome and now married to an Italian diplomat.
After Marjorie's death in 1996, he married Ania Czepulkowska, a young Polish artist holding a Royal College Masters degree, in 2000.
Since his retirement he has written political books based on his interest in psychoanalysis.
This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.