Violent clashes between Hong Kong riot police and students have galvanised tens of thousands of supporters for the city's pro-democracy movement who continue to occupy the heart of the city's financial centre.

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.

Occupy is demanding that China 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 China 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.

Al Jazeera's Rob McBride, reporting from the protest site on Sunday, said the environment was tense in the square as demonstrators continued their sit-in action and police barricades were kept in place.

"The movement has grown by thousands in hours. Things have grown faster than anticipated by anybody. Protesters are waiting for the reaction of the Beijing government."

Act of civil disobedience

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.

Whoever loves Hong Kong should come and join us. This is for Hong Kong's future. Jimmy Lai, publishing tycon

The 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.

AFP news agency said that 74 people had been arrested so far by the city's police.

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.

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.

"Whoever loves Hong Kong should come and join us. This is for Hong Kong's future," Jimmy Lai, a publishing tycoon, told Reuters news agency.

Lai, an outspoken critic of China's communist government, has backed pro-democracy activists through publications that include one of the city's biggest newspapers as well as donations.