'It's a great loss for the country. He really believed in democracy and never stopped fighting for Singapore,' his assistant Ng Teck Siong said.
In recent years, Mr Jeyaretnam - once a wealthy, flamboyant and high-profile lawyer - had stood on street corners and outside subway stations to pedal his own books about Singapore politics because no retailer would stock them.
The book sales were meant to raise money to help pay off damages stemming from defamation suits Mr Jeyaretnam lost to Prime Minister Lee Hsien Loong, Mr Lee's father and Singapore's founding leader Lee Kuan Yew, former Prime ministers Goh Chok Tong, and others.
'It's a very heavy price I have paid' for taking on the government, Mr Jeyaretnam, the first opposition member to be elected to Parliament, told The Associated Press recently.
The government did not immediately respond to Mr Jeyaretnam's death.
Mr Jeyaretnam served as a member of parliament from 1981 to 1986 and from 1997 to 2001 for the Workers' Party, which he founded.
He left the party in 2001 and helped form the Reform Party this year. He was planning to run in the next parliament election, due by 2011.
Mr Jeyaretnam, whose thick white whiskers and misty eyes made him instantly recognisable, often faced jeers and catcalls in Parliament from the ruling People's Action Party, whose members have always vastly outnumbered the opposition.
At present, the opposition holds two out of 84 elected seats in Parliament.
The PAP has ruled Singapore since independence from Malaysia in 1965.
A socialist at heart, Mr Jeyaretnam contended that the government's economic policies created a wealthy upper class and an underbelly of poor citizens who have to work twice as hard to survive.
He also often railed against what he called the 'Lee dynasty', a reference to Minister Mentor Lee Kuan Yew and his prime minister son.
His views inevitably got him into trouble with the Lees and other government leaders who frequently sued him for defamation.
He said he had lost count of how many times he had been sued - and lost.
He estimated that he paid out more than S$1.6 million in damages and court costs over the years.
After losing the last defamation case, Mr Jeyaretnam declared bankruptcy in 2001.
He was found guilty of defaming them at a 1997 election rally, when he said a colleague had filed a police report accusing the ruling party leaders of defamation.
An Anglican Christian of Sri Lankan Tamil decent, Mr Jeyaretnam attended Saint Andrew's School in Singapore and University College London where he earned a bachelor's degree in law.
His wife, Margaret, whom he had met when they were law students in Britain, died of breast cancer a year before he was elected to Parliament in 1981.
He is survived by two sons, Kenneth and Philip. The funeral will be held on Thursday.