Pro-democracy activists including high-profile student campaigner Joshua Wong draped a black flag over a statue symbolising Hong Kong's return to China in an early morning act of defiance.
Dozens more campaigners marched to China's representative office in the city in the afternoon in a rally against the detention of hundreds of human rights lawyers on the mainland, while a group of protesters also gathered in central Hong Kong to "sing for democracy".
Xi's visit will be his first since becoming president in 2013 and will culminate with the inauguration of Hong Kong's new leader, Carrie Lam, on Saturday.
It comes as there are increasing concerns Beijing is trampling the handover agreement guaranteeing Hong Kong's semi-autonomous status by interfering in a range of areas, from politics to education and media.
In the morning demonstration, a dozen activists attached a black cloth to the giant golden bauhinia flower on Hong Kong's harbourfront as security tried to stop them climbing on the famous tourist attraction.
The sculpture of the bauhinia, which became the emblem of Hong Kong after the handover, was a present to the city from China in 1997 and stands outside the convention centre where Xi will attend anniversary events during a three-day visit starting Thursday.
Police were called to take the flag down while the protesters chanted "democratic self-determination for Hong Kong's future" and "one country, two systems has been a lie for 20 years", referring to Hong Kong's semi-autonomous status.
Campaigners like Wong are calling for democratic reforms, promised in the handover deal, to change a system where the city leader is still chosen by a pro-China committee and the legislature is weighted towards Beijing.
Since then calls for self-determination or even full independence from China have emerged.
Wong's party Demosisto wants a public referendum on Hong Kong's future in 2047, the year the handover agreement protecting the city's way of life and liberties expires.
Veteran politician and former legislator Emily Lau was among those who gathered in a separate protest Monday afternoon against the detention of lawyers by China, which she called "barbaric".
"We firmly believe that a regime that does not respect basic human rights... cannot maintain stability and cannot secure legitimacy," added activist Joseph Cheng.
Another group of around 20 protesters, including pro-democracy lawmaker Claudia Mo, sang "Do You Hear the People Sing?" outside the Court of Final Appeal in central Hong Kong in the late afternoon.
The song from the musical "Les Miserables" was a popular chant for the tens of thousands who gathered during the 2014 Umbrella Movement which brought parts of Hong Kong to a standstill for over two months.
Protesters say they are preparing further actions during the handover celebrations and Xi's visit will be shrouded in a huge security operation.
(Except for the headline, this story has not been edited by NDTV staff and is published from a syndicated feed.)