As many as 800 activists were eventually smuggled out of China, including those at the top of China's most-wanted list - such as the student leaders Wu'er Kaixi and Chai Ling, and high-profile government officials such as Yan Jaiqi, who was an adviser to the deposed Communist Party leader Zhao Ziyang.
Britain's role in the escapes has been kept under wraps by the dissidents themselves and the British government, which sought assurances of their silence in return for help, lest Hong Kong was penalised for giving refuge. However, a decade after the massacre, some of those involved have now spoken to The Independent about what happened.
Shortly after the crackdown in Peking on 4 June 1989, the round up of pro-democracy activists spread to other parts of the country. It was clear to leaders of the democracy movement they had to escape and that the only realistic exit point was British-controlled Hong Kong. The leaders of Hong Kong's large pro-democracy movement agreed, and with remarkable speed they established an underground conduit out of China, which was known as Operation Yellowbird.
The Hong Kong organisers gathered substantial sums of money to finance their effort, a lot of which was spent bribing Chinese officials and paying off Triad "snake heads" who did the actual smuggling.
The problem now was to persuade the British authorities to admit the dissidents and allow them to stay in Hong Kong while preparations were made for them to leave for countries of asylum - the United States and France took most by far. This involved close British liaison with local diplomatic missions and driving a cart and horse through Hong Kong's immigration rules.
The Yellowbird organisers formed tentative contacts with the Hong Kong government and responsibility for co-ordinating the escapes was quickly passed over to the Political Adviser's office, a special unit working closely with the Governor. The advisers were all diplomats on secondment from the Foreign Office, not part of the Hong Kong government structure.
The office was headed by William Ehrman, who is now ambassador to Luxembourg. Sir David Wilson, who was then the Governor, gave his active support to the escape effort and London was kept informed of what was happening.
Mr Ehrman's team established a task force with the Hong Kong security services to ensure that the dissidents could enter and leave Hong Kong unobtrusively, bypassing the normal immigration controls. This they arranged through a link man in the normally highly bureaucratic Security Branch, an expatriate British former policeman.
The Chinese authorities were well aware that the dissidents were seeping through Hong Kong but turned a blind eye. As one British official put it: "We sensed they were pleased to see them go and ... were also pleased that we gave them no cause to confront the issue."
One of the Hong Kong Yellowbird organisers interviewed for this article said: "I had not really thought about the role Britain played until you asked me but the truth is that had we not had Hong Kong as an escape route these people would have been trapped. Britain deserves a lot of thanks for making sure that did not happen."