Joshua Wong was among around 30 protesters who had staged a three-hour sit-in at a harbourfront statue and were led away into police vans.
Xi's visit this week marks 20 years since Hong Kong was handed back to China by Britain and comes at a time when fears are growing that Beijing is tightening its grip on the semi-autonomous city.
Protesters had encircled the sculpture of a golden bauhinia flower which became the emblem of Hong Kong in 1997.
The statue was given to the city by China as a present to mark the handover.
Some activists chained themselves to the sculpture while others climbed into its petals.
Police cleared the area and surrounded the statue, leading away protesters one by one.
Most walked but Wong and young legislator Nathan Law lay down and were carried away from the scene.
Before the arrests began, protesters shouted: "Civil disobedience, no fear!" and "Xi Jinping, can you hear us?"
Police had warned them that they were causing a public nuisance and would be arrested if they did not move.
As Wong was carried away to the awaiting vans, he shouted: "Protest on July 1!" - the anniversary of the handover.
His party Demosisto, which is calling for self-determination for Hong Kong, said in a statement that eight of its activists, including Wong and Law, had been arrested.
Pro-democracy activists hang a black cloth which reads 'Release Liu Xiaobo unconditionally, Hong Kong people want genuine universal suffrage', June 28, 2017. Source: AP
The final protesters to be removed were inside the golden flower and helped down by firefighters.
'I'm a Hong Konger!'
Xi is due to land in Hong Kong on Thursday for a three-day visit to attend anniversary celebrations and swear in the city's new leader Carrie Lam.
Hong Kong is ruled under a "one country, two systems" deal, enshrined in the handover agreement, which allows it rights unseen on the mainland, including freedom of speech and an independent judiciary.
But there are concerns that China is increasingly interfering in a range of areas, from politics to education and the media.
Protesters at the bauhinia statue chanted "Long live the Umbrella Movement!" and "I'm a Hong Konger!"
The Umbrella Movement was the name given to mass rallies in 2014 calling for democratic reforms. Wong and Law were among the student leaders of those protests, which ultimately failed to win concessions.
Frustration over the lack of progress has prompted campaigners like Wong and Law to call for self-determination - other activists are demanding a complete split from China.
Protesters at the statue also called for the release of Chinese dissident Liu Xiaobo who was granted medical parole this week due to late-stage liver cancer but remains in the mainland.
The writer and Nobel Peace Prize laureate, now 61, was sentenced to 11 years in prison in 2009 for "subversion"after spearheading a bold petition for democratic reforms.
The bauhinia statue stands outside the convention centre where Xi will attend anniversary events and is a stone's throw from the hotel he will stay in.
The spot is popular with mainland visitors.
Some of the demonstrators, including Wong, had already draped the statue with a black flag on Monday during an early morning anti-China protest. It was removed by police.
