Since Hong Kong switched from British to Chinese rule in 1997, Lee and the party he helped found have been a thorn in Beijing's side.
Lee began his political career in 1985 as a lawmaker for the legal constituency.
From then until 1989 he was a member of Hong Kong's Basic Law drafting committee, but quit the body following the Tiananmen Square crackdown on June 4, 1989.
Lee has been banned from visiting China since the 1980s for criticising human rights conditions there.
Lost democracy dream
He told Reuters on Friday that his biggest regret throughout his career had been that he "could not realise the democracy dream".
At the end of last year, China's parliament made a landmark ruling to possibly allow direct elections for Hong Kong's leader in 2017, although key details have been left vague.
Lee's prominent role as a democracy activist has drawn fire from Hong Kong's considerable pro-Beijing camp.
A jeering crowd heckled him outside the legislature after he wrote an article in The Wall Street Journal in October last year, stating that Beijing's Olympic Games provided a good opportunity to push for human rights improvements in China.
Numerous articles in the Hong Kong press denounced him, while the pro-Beijing Sing Tao newspaper called him a “traitor.”