The appointment of Mr Lee - a 64-year-old former security chief who got the job in a one-man election - has worried activists.

This is a man most commonly known for overseeing the police's violent response to mass pro-democracy protests in 2019 and who has been sanctioned by the US over rights abuses.

With pro-Beijing views, many pro-democracy activists have raised concerns over what his leadership will mean for their movement and the future of the former British colony.

'Notoriously difficult to engage with'

Mr Lee began in law enforcement and joined Hong Kong's police force in 1977, aged 20.

Since then he has risen through the ranks and was eventually appointed as security chief under Carrie Lam's administration.

Former student leader and pro-democracy activist Nathan Law has met Mr Lee and described him as "authoritarian" and a more "hawkish" leader than Ms Lam.

The 28-year-old campaigner, who is wanted on several charges in China and Hong Kong and is living as a political refugee in the UK, told Sky News Mr Lee is "notoriously difficult to deal with".

"He was a very authoritarian person who feels like as long as he has the power, he doesn't have to explain things or be held accountable," he said.

"So it's not a surprise that Beijing will pick him."

Mr Law added that he thinks Mr Lee's appointment is "problematic" for the pro-democracy movement, given his tough stance on protesters.

Violent police crackdown

As security chief, Mr Lee led the police force during the months-long pro-democracy protests in 2019, which were sparked by a controversial bill that would have allowed people accused of certain crimes to be sent to China for trial.

Police fired rubber bullets, water cannons and tear gas against protesters and were internationally criticised for their forceful response.

Thousands of people have been jailed for offences linked to the protests.

Sanctioned by the US

Following the protests, Mr Lee's close relationship with Beijing continued with his support for the national security law - the authorities' answer to the protests.

The controversial law, which came into force in 2020, made it easier for authorities to crackdown on dissent and to punish crimes with possible life imprisonment.

It has seen people arrested for offences such as subversion, secession, terrorism, and collusion with foreign forces.

After the law was passed, Mr Lee was sanctioned by the US for "undermining Hong Kong's autonomy".

Mr Lee responded: "Some bullying countries tried to use sanction measures to scare officials.

"That makes us more persistent to carry on maintaining national security."

Concerns for the future of Hong Kong

While Hong Kong's 7.4 million people still have greater freedom than those in mainland China, in recent years Beijing has tightened its grip on the territory.

Although it is ruled by a 'one country, two systems' principle which should allow some autonomy, critics say this is eroding.

With the national security law, the crackdown on opposition and the "election" of a pro-Beijing hardliner - pro-democracy activists fear further restriction under Mr Lee's leadership.

Mr Law said the movement has gone "more underground" since the introduction of the national security law.

He added that he thinks there has been a "spike" in people starting to plan to leave Hong Kong over fears of Mr Lee's impact on freedom of speech.

International criticism over Lee's 'election'

Concerns over Mr Lee's leadership extends beyond Hong Kong, with international figures and leaders also criticising the process in which he was elected.

Chief executives are chosen by a 1,500-member election committee, most of whom are pro-Beijing loyalists.

In response to Mr Lee's victory, EU's foreign policy chief Josep Borrell described the selection process as "another step in the dismantling of the 'one country, two systems' principle".

As leader, Mr Lee has vowed national security will be his "fundamental mission" while in office, as well as restoring the territory's international status.

He has described his cabinet, filled with officials with a background in security, as "results oriented", hoping to usher in a new chapter following the 2019 protests.