Hong Kong students mobbed the city's leader Tuesday in angry scenes as they took their anti-Beijing strike to government headquarters, where more than 1,000 protested against China's refusal to grant full democracy.

Organisers said 13,000 university students massed at a campus in the north of the semi-autonomous city on Monday to launch a week-long boycott of classes, a strong showing that breathed new life into the democracy campaign which had been stunned by Beijing's hardline stance.

Activists say the student protest marks the start of a campaign of civil disobedience against China's plan to vet nominees for the leadership of the former British colony, dashing hopes for full universal suffrage at 2017 polls.

About a dozen students rushed towards Hong Kong's leader, chief executive Leung Chun-ying, on Tuesday when he emerged from government headquarters after holding a press conference.

Leung was hustled away while security officials forced the students back, escorting them from the grounds as dozens of journalists joined the melee.

"This is a warning. Your actions have already severely disrupted order here," police said over loudspeakers, while in response protesters chanted: "Hong Kong belongs to us!"

Leung told the press conference that authorities had "paid close attention to the demands for the election in 2017 by the university students" and that Beijing's proposals were an improvement on the current state of democracy in Hong Kong, whose leader is currently appointed by a pro-government panel.

"You can see that he has no intention of having a dialogue with the students," said Alex Chow, chairman of the Hong Kong Federation of Students and one of the activists who challenged Leung.

Chow threatened an escalation of the protest action if Leung refuses to speak with the students within 48 hours.

- Dissidents warn of 'second Tiananmen' -

The park outside the Hong Kong government's Tamar headquarters took on a carnival atmosphere as protesters trickled in under the summer sun, to attend a programme including lectures on the lawns.

"The government officials, the legislators, they can look out their window and see us calling for true democracy," 20-year-old political science student Ester Wong told AFP.

"This park has a lot of significance in Hong Kong protest movements, and we're here to continue that tradition," she said, huddled with others under the shade of trees and tents.

The students are heartened by past successes, including the government's 2012 climbdown on a plan to implement patriotism classes which was abandoned in the face of mass protests outside government headquarters.

"Someone needs to take the lead in showing the government they're wrong, and this time it's up to the students," said 19-year-old theatre student Ryan Lo.

"We oppose the Chinese government trying to limit the freedoms Hong Kong people deserve."

Organisers said 4,000 people attended Tuesday's daytime activities outside the government headquarters, which are set to last until Friday, with numbers expected to ebb and flow throughout the week.

Chinese political dissidents including Beijing-based Hu Jia called on the international community to take action to prevent a repeat of the bloody crackdown on the student-led Tiananmen Square protests.

"As widespread demonstrations grow against Beijing's violation of its promise to allow universal suffrage, there is a danger the infamous 1989 massacre in Tiananmen Square could be repeated in Hong Kong," they said in an opinion piece in the Wall Street Journal.

"The United States and the international community share the responsibility to prevent another murderous massacre," they said.

A coalition of pro-democracy groups, led by Occupy Central, have labelled the election restrictions a "fake democracy" and have vowed a series of actions including a blockade of the Central financial district.

Britain handed Hong Kong back to China in 1997 under a 'one country, two systems' agreement which allows civil liberties not seen on the mainland, including free speech and the right to protest.

But tensions have been growing in the southern Chinese city over rising inequality and Beijing's perceived political interference.

Trust in the city's government fell to its lowest this month since 2003, according to a Hong Kong University survey released Tuesday, with 43 percent saying they distrust Leung's administration.