By James Pomfret and Yimou Lee

HONG KONG, Sept 28 (Reuters) - Violent clashes between Hong Kong riot police and students galvanized tens of thousands of supporters for the city's pro-democracy movement and kick-started a plan to lock down the heart of the Asian financial center early on Sunday.

Leaders and supporters of Occupy Central with Love and Peace rallied to support students who were doused with pepper spray early on Saturday after they broke through police barriers and stormed the city's government headquarters.

"Whoever loves Hong Kong should come and join us. This is for Hong Kong's future," publishing tycoon Jimmy Lai, an outspoken critic of China's communist government who has backed pro-democracy activists through publications that include one of the city's biggest newspapers as well as donations, told Reuters.

Occupy demanded that Beijing withdraw its framework for political reform in the former British colony and resume talks.

Hong Kong returned to Chinese rule in 1997 under a formula known as "one country, two systems." that guaranteed a high degree of autonomy and freedoms not enjoyed in mainland China. Universal suffrage was set as an eventual goal.

But Beijing last month rejected demands for people to freely choose the city's next leader, prompting threats from activists to shut down Central, Hong Kong's financial district. China wants to limit elections to a handful of candidates loyal to Beijing.

"It's high time that we really showed that we want to be free and not to be slaves ... we must unite together," Cardinal Joseph Zen, 82, formerly Catholic Bishop of Hong Kong, told Reuters.

This demonstration, which has drawn thousands of protesters armed with goggles, masks and raincoats in preparation for a violent confrontation with police, is one of the most tenacious acts of civil disobedience seen in post-colonial Hong Kong.

Roads in a square block around the city's government headquarters, located in the Admiralty district adjacent to Central, were filled with people and blocked with metal barricades erected by protesters to defend against a possible police crackdown.

Some of Hong Kong's most powerful tycoons have spoken out against the Occupy movement, warning it could threaten the city's business and economic stability.

The latest protests escalated after demonstrators broke through a cordon late on Friday and scaled perimeter fences to invade the city's main government compound in the culmination of a week-long rally to demand free elections.

Student leaders said about 80,000 people participated in the rally. No independent estimate was available.

TENSIONS ESCALATE

The clashes were the most heated in a series of anti-Beijing protests that underscore the central government's challenge to stamp its will on Hong Kong.

Some observers have likened the protests to those that culminated in the bloody crackdown on pro-democracy students in and around Beijing's Tiananmen Square in 1989.

Police arrested more than 60 people, including Joshua Wong, the 17-year-old leader of student group Scholarism, who was dragged away after he called on the protesters to charge the government premises. He was still being detained early on Sunday, along with fellow student leaders Alex Chow and Lester Shum.

His parents said in a statement the decision to detain him was an act of "political persecution."

Wong has already won one major victory against Beijing. In 2012, he forced the Hong Kong government to shelve plans to roll out a pro-China national education scheme in the city's schools when the then 15-year-old rallied 120,000 protesters.

Students issued rallying cries during the protests, calling for their leaders' release. But divisions between the students and Occupy quickly emerged as arguments broke out and some students accused the civil disobedience movement of hijacking their protest.

"I came here tonight to support the students, but now I feel like I've been used ... They made that decision without asking us," said Sharon Choi, 20.

Occupy organizers had previously indicated they planned to blockade the financial district on Oct. 1, China's National Day holiday. The rally will now take part in the Admiralty district to build on the momentum of week-long student rallies and protests in the area.

"Rather than encouraging the students to join, we are encouraged by the students to join," said Benny Tai, one of the three main organizers of the pro-democracy movement.

"We are touched and moved by the work of the students." (Additional reporting by Stefanie McIntyre, Venus Wu, Diana Chan, Kinling Lo, Donny Kwok, Farah Master, Charlie Zhu, Twinnie Siu and Bobby Yip; writing by Anne Marie Roantree, editing by G Crosse)