In the handover agreement, ratified in 1985, Britain agreed to handover Hong Kong in 1997, and the agreement allowed for One Country, Two Systems, and Hong Kong's way of life under the U.K.to continue for 50 years.
But while Hong Kong's Basic Law guarantees wide-ranging freedoms under the "one country, two systems" formula, Beijing's refusal to grant universal suffrage has triggered sometimes violent unrest.
Others demanded that Hong Kong's outgoing chief executive, Leung Chun-ying, be locked up over corruption allegations, and many slogans were simply words of encouragement for a movement that has seen little progress on democracy: "Never back down", "Fight for Hong Kong", and, more poetically, "Being born in uncertain times carries certain responsibilities".
However, ideologies held by localist groups and pro-Hong Kong independence groups are not the same, and the ideas people hold regarding independence change rapidly, Chan said.
"Any attempt to endanger China's sovereignty and security. or use Hong Kong to carry out infiltration and sabotage activities against the mainland is an act that crosses a red line", Xi said, in a nationalistic speech which emphasized Beijing's control over the city, now a special administrative region of China.
A proposal to include patriotic education in Hong Kong schools met with huge protests in 2012 and has since been shelved.
"Education on national history and culture needs to be strengthened", Xi said. "The UK has no sovereignty, no power to rule and no power to supervise Hong Kong".
Protesters waved placards denouncing Hong Kong and Beijing leaders and banners in support of a wide range of causes. "What we've experienced this weekend was a whole new level of intimidation and direct violence that's unprecedented", said campaigner Avery Ng of League of Social Democrats.
With last year's protest at 110,000, as estimated by organisers Civil Rights Fronts, this year saw a reported 60,000 take to the streets, on a day that coincided with the end of the contentious three-day visit of China's state leader Xi Jinping.
Participants said the anniversary also comes amid widespread public anger at Hong Kong's own government, which has frequently been accused of caving in too easily to Beijing's interference in the city in recent years.
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Many Hong Kong residents, anxious about growing encroachment by Beijing as the financial hub marks 20 years since its return to China, are rushing to secure British passports as a safety net in the case of social unrest or the erosion of civil liberties. As soon as he landed on Hong Kong soil, the Communist leader promised to work for a "two-country system" to remain "stable and for a long time" and congratulate local authorities on their "commitment" to fighting the independence forces. Liu's face featured on countless signs held aloft by marchers who called on China to release him to seek treatment overseas.
"It's not straightforward to get what we want by just protesting, especially from this heartless government", said teacher Charlton Cheung, in his 40s.
He also said that Hong Kong should always focus on development as the top priority.
Thousands of protesters joined the rally organized by a pro-democracy coalition at its starting point in downtown Victoria Park.
"The Chinese government has treated him inhumanely, and persecuted his wife and those around him in all manner of illegal ways", he said. "The Communist party's solution to the world's problems is money, because that is all they have", said Martin Lee, a veteran activist and former legislator. Lam called for unity to heal the city's divided society - just hours after police scuffled with pro-democracy protesters attempting to reach the harbor-side venue where the ceremony took place.
"It's totally a failure of one country, two systems. if our peaceful demonstration, the right to assembly is eroded by those triads", he said, saying he should never have been handcuffed. Leading activist Joshua Wong accused "pro-China gangsters and mobs" for instigating the attack.
Ron Wong, 17, who was marching with his parents, said Xi's visit had been a "show of power of who's in charge".
An online video posted by the party on its Twitter account appeared to support their claim. Progress in the "integration" of China and Hong Kong in the economic field seems to sustain the tough stance of the Xi administration.
"All my land has been taken away, and my home was demolished", Jiang said.
Reported by Lau Siu-fung and Goh Fung for RFA's Cantonese Service, and by Qiao Long for the Mandarin Service.