* Xi says Hong Kong to maintain "one country, two systems"

* Critics say Beijing has curtailed city's autonomy

* New leader John Lee sworn in; replaces Carrie Lam (Adds more Xi quotes, activist, background)

By Meg Shen and James Pomfret

HONG KONG, July 1 (Reuters) - There is no reason to change Hong Kong's "one country, two systems" formula of governance, Chinese President Xi Jinping said on a rare visit to the global financial hub after swearing in the city's new leader, John Lee, on Friday.

Britain returned Hong Kong to Chinese rule on July 1, 1997, with Beijing promising wide-ranging autonomy, unfettered individual rights and judicial independence at least until 2047.

China's critics accuse authorities of trampling on those freedoms, unavailable on the authoritarian mainland, with a sweeping national security law imposed by Beijing on the city in 2020 after mass pro-democracy protests the year before.

British Prime Minister Boris Johnson and U.S. Secretary of State Antony Blinken said on Thursday that China had failed to meet its handover commitments.

China and Hong Kong reject the accusations, saying the law "restored order from chaos" so that the city could prosper.

Xi said the "one country, two systems" formula was successful under China's "comprehensive jurisdiction."

"For this kind of good system, there is no reason at all to change it. It must be maintained over the long term," Xi said.

"After experiencing wind and rain, everyone can painfully feel that Hong Kong cannot be chaotic, and must not become chaotic again ... Hong Kong’s development cannot be delayed again, and any interference must be eliminated."

Xi added China would support Hong Kong's role as an international finance and trade hub.

At the swearing-in ceremonies, all officials, including Xi, wore masks and did not shake hands.

Former police officer Lee, sanctioned by Washington over his role in implementing the security law, takes charge as the city is facing an exodus of people and talent amid some of the toughest COVID-19 restrictions in the world.

Authorities deployed a massive security force, blocking roads and the airspace around the picturesque Victoria Harbour, where the last colonial governor, Chris Patten, tearfully handed Hong Kong back to China at a rain-drenched ceremony in 1997.

Red lanterns, Chinese and Hong Kong flags, and posters declaring a "new era" of stability decorated districts across the city.

Xi did not attend the traditional flag-raising ceremonies on Friday, with media reporting he stayed overnight across the border in Shenzhen after arriving in Hong Kong on Thursday.