By Venus Wu

HONG KONG (Reuters) - Tens of thousands of Hong Kong residents marched in protest on the 19th anniversary of the financial hub's return to Chinese rule on Friday as tensions simmer against Chinese authorities over the abductions of Hong Kong booksellers.

Some waved banners criticizing Beijing for the cross-border abductions as acts of a "totalitarian" regime, as well as calling for the release of leading dissidents, chanting for democracy and for Hong Kong leader Leung Chun-ying to step down.

Several hundred scuffled with police outside Government House, with police using pepper spray to keep them back. Organizers said 110,000 people took part in the march, while police put the figure at 19,300.

The July 1 protests are considered a barometer of public sentiment toward Beijing, with the former British colony due to hold citywide elections in September.

The city has been unnerved over the past year by the disappearances of five booksellers who specialized in works critical of Chinese leaders. One of the men, Lam Wing-kee, who was detained for eight months by Chinese agents and released last month, criticized Beijing for "violating Hong Kong's rights" through illegal cross-border enforcement operations.

The tactics have raised fears of Communist Party rulers in Beijing eroding the so-called "one country, two systems" formula, granting Hong Kong a high degree of freedom and autonomy since its 1997 return from British to Chinese rule.

China has denied wrongdoing.

"This is a very grave threat to the safety of Hong Kong residents that an unknown force is spying on people," said pro-democracy lawmaker Cyd Ho at the rally.

"The Hong Kong government has to follow up with the central government on what's really happening behind the scenes."

Hundreds of police were also deployed to guard China's main representative "Liaison Office" in Hong Kong, after activists who advocate independence from China posted plans on social media for a "black mask" evening protest to besiege the skyscraper.

Scores of young people, some dressed in black T-shirts with the words "HK is not China", were searched by police in the area and roads were blocked off with metal barricades to prevent trouble.

Lam, who had been due to lead the July 1 march that each year draws tens of thousands, pulled out, citing safety concerns after being followed by two unknown strangers, a lawmaker said.

"He feels increasingly concerned about his own personal safety," said pro-democracy lawmaker Albert Ho.

A senior Chinese official, Wang Guangya, who heads the Hong Kong and Macau Affairs Office in Beijing, said the booksellers had "destroyed" the one country, two systems formula by publishing banned books in mainland China.

Chinese President Xi Jinping, however, said in a speech on Friday that "no matter what the difficulties and challenges, our confidence and determination towards one country, two systems will not waver".

Xi added Hong Kong would continue to enjoy a high degree of autonomy and Beijing would strictly adhere to the law.

A 79-day "umbrella revolution" in late 2014 demanding Beijing allow full democracy in Hong Kong brought chaos to the streets.

(Additional reporting by Lindsy Long, Sharon Shi and Hera Poon in Hong Kong and Ben Blanchard in Beijing; Writing by James Pomfret; Editing by Nick Macfie)