A flash mob sprang up in the heart of Hong Kong's financial centre on Tuesday, hours after police fired tear gas at a university campus and territory-wide transport disruptions wreaked commuter havoc in the Chinese-ruled city.

More than 1,000 protesters, many wearing office clothes and face masks, rallied in the Central business district for a second day, blocking roads below some of the city's tallest skyscrapers and most expensive real estate.

Some crouched behind umbrellas as riot police stood by.

Others chanted "Fight for freedom, stand with Hong Kong" and "Five demands, not one less", referring to calls for universal suffrage and an independent probe into perceived police brutality, among other demands.

"Let's stop focusing on the result. The result is all this chaos and madness. But what is the root cause? We don’t have a fully democratic system," said one demonstrator, a 25-year-old property manager wearing a white shirt, black suit pants and a black face mask who gave his name as Roy.

"We want to enjoy air-conditioning and a beer with friends. No Hong Konger wants this (violence), but the government is forcing us to take things to such a dramatic (level)."

Police on Monday fired volley after volley of tear gas in Central, where some protesters blocked narrow streets lined with banks, top-brand shopping malls and jewellery shops. Most had pulled down their shutters on Tuesday.

Emily, in her 30s and working in the finance sector, was carrying a black leather shoulder bag and wearing a black mask and swimming goggles on the front line on Tuesday.

In the bag was a bowl to cover tear gas canisters when they land on the street and a gas mask.

"I won’t take part in the attacks, I am here to try to protect the kids," she told Reuters.

Tension eased as the lunch hour ended, but many people milled around the streets, in between the bricks and barricades made of debris. Riot police, who earlier raised black flags warning of tear gas, had disappeared.

In an ominous sign, the Chinese University in the New Territories said some people had broken into a storeroom and made off with bows, arrows and javelins. All were later retrieved, it said.

There was chaos earlier as people thronged metro stations only to stream out again after some train services were suspended.

Some roads were closed with long traffic jams building during rush hour, a day after some of the worst violence to rock the former British colony. A protester was shot by police and a man set on fire on Monday.

Universities and schools cancelled classes, with students, teachers and parents on edge a day after police fired tear gas and students hurled petrol bombs on campuses.

Lam said protesters were being extremely selfish and hoped that universities and schools would urge students not to take part in the demonstrations.

More than 260 people were arrested on Monday, police said, bringing the total number to more than 3,000 since the protests escalated in June.

Protesters are angry about what they see as police brutality and meddling by Beijing in the freedoms guaranteed under the "one country, two systems" formula put in place when the territory returned to China in 1997.

China denies interfering and has blamed Western countries, including Britain and the United States, for stirring up trouble.

- Reuters