The abduction of several Hong Kong booksellers, who sold publications critical of China's leaders, by mainland agents has also shaken confidence in Beijing's promise of non-interference, they say.
And he warned that anyone threatening China or Hong Kong's political stability would be crossing a red line and their actions would be considered "absolutely impermissible"- words certain to concern those already wary of tightening restrictions on political life in the city.
The Sino-British Joint Declaration gave Hong Kong rights unseen on the mainland through a "one country, two systems" agreement, lasting 50 years.
Like many of his peers, Ngan plans to renew his BN (O), or British National (Overseas), passport, a special status passport granted to Hong Kong permanent residents prior to 1997.
He took part in a series of lavish events, including a flag-raising ceremony, amid tight police security.
"Suppression by the regime has never let up and I don't see any end in sight", said activist Nathan Law, Hong Kong's youngest legislator, who was arrested Wednesday over an anti-China sit-in and attended the protest after being released Friday.More news: Venus Williams blamed in fatal Palm Beach Gardens crash
President Xi travelled through the Hong Kong Link Road (HKLR) Scenic Hill Tunnel, which was connected in May this year, to inspect the HZMB HKLR.
Earlier in the day, Lam Cheng Yuet-ngor was sworn-in as Hong Kong's new Chief Executive, becoming the territory's first female leader.
The U.S. State Department also said Thursday that the USA "remains concerned about any infringement of civil liberties in Hong Kong", and expressed support for the "further development of Hong Kong's democratic systems".
Beyond democracy, many protesters said they were also protesting about Hong Kong's many social problems, from staggering income inequality to corruption in government.
China pledged to ensure "a high degree of autonomy" that broadly accommodates the uniqueness of Hong Kong under the "one country, two systems" policy.
But the march, which saw banners with the slogans "Democracy Delay No More!"More news: Saints place Nick Fairley on NFI list, will miss entire season
He said Hong Kong continues to serve as an global financial, trade and shipping hub, and the region's indicators for political stability, government effectiveness, regulatory quality, rule of law and corruption control are much higher than they were before 1997.
Reports of clashes between protesters and local gangsters, alongside allegations of attacks from police, marked an otherwise subdued and non-violent affair in which placards borrowing the symbols of 2014's Umbrella Movement were wielded throughout.
A vendor sells cushions depicting the Apple Inc logo and a British passport inside a Lunar New Year market at Hong Kong's Victoria Park February 17, 2015.
In response to the USA and United Kingdom statements, Chinese foreign ministry spokesman Lu Kang detailed Hong Kong's "comprehensive achievements with the support of the central government and mainland".
China's President urged Hong Kong to do more to increase security and boost "patriotic education".
And only in Hong Kong bookstores can one find writings by and in praise of taboo figures like the Dalai Lama and Liu Xiaobo, the jailed Nobel laureate.
The plan was voted down by pro-democracy lawmakers and the reform process has since stalled, with Lam making no commitment to revisit it anytime soon.More news: Sheriff: Man killed missing son, 5, after Disneyland trip