Dressed in white and wearing yellow ribbons, students from more than 20 universities and colleges packed into the grounds of picturesque, bay-side Chinese University where they were greeted by banners that said: "The boycott must happen. Disobey and grasp your destiny."
Managing the former British colony is proving a challenge for Beijing, which is worried that calls for democracy could spread to cities on the mainland, threatening the Communist Party's grip on power.
Hong Kong returned to Chinese rule in 1997 as a "special administrative region" (SAR) with a high degree of autonomy and freedoms not enjoyed on the mainland under a formula known as "one country, two systems".
But Beijing last month rejected demands for people to freely choose the city's next leader, prompting threats from pro-democracy activists to shut down the Central financial district.
"We are willing to pay the price for democracy. No one can take away people's entitled right. No one. Not the SAR government and of course not the NPC," said Alex Chow, leader of the Hong Kong Federation of Students, one of the organisers of the boycott, referring to China's National People's Congress.
Chow, 24, who wore a black T-shirt with the words "freedom now", has said he was inspired by a high school teacher who began crying as he played a clip of China's bloody crackdown on pro-democracy student demonstrators in and around Beijing's Tiananmen Square on June 4, 1989.
The protest was peaceful but the mood at the university was defiant as demonstrators demanded nominations for Hong Kong's next leader in 2017 to be open to everyone. China's leaders want to ensure only pro-Beijing candidates are on the ballot.
Chow's federation put the number of students attending the rally at about 13,000, describing the turn-out as "inspirational". There was no independent or police estimate.
The Occupy Central movement that has threatened to shut down the business district will likely be encouraged by the turnout. The students' ability to mobilise such a large crowd makes their support an increasingly important driver of Hong Kong's burgeoning civil disobedience movement.
Students converged on a long boulevard at the university, with some carrying umbrellas to protect them from the baking sun, and many chanting "united we stand" and "democracy now".
The Federation of Students said its application to hold a rally in Hong Kong's Central district had been approved for Tuesday to Thursday.
"NORMAL PHENOMENON"
It had written a letter to Hong Kong leader Leung Chun-ying and planned to gather outside his office on Tuesday if it had not received a reply by then.
The Hong Kong government said in a statement it respected the students' "ambition and persistence".
"The issue of political system development has been complicated and controversial, so it's understandable that different groups in the society hold different opinions and arguments, which is also a normal phenomenon in Hong Kong's diversified society," it said.
The student boycott coincided with a trip by some of Hong Kong's most powerful tycoons to Beijing where they discussed Hong Kong with Chinese leader Xi Jinping.
"We will continue to carry out 'one country, two systems' and the Basic Law in Hong Kong, which serve the interests of the nation, the interests of Hong Kong people, and the interests of foreign investors," said Xi, who was flanked by former Hong Kong leader Tung Chee-hwa.
"The central government will firmly support and push for the democratic development in Hong Kong and will maintain the prosperous development in Hong Kong."
The Basic Law refers to the mini-constitution for post-1997 Hong Kong which enshrines the one country, two systems formula.
Leading academics in Hong Kong have voiced support for the student boycott, with some offering to record lectures and post them online for students who miss school to watch later.
"As long as the spirit of democracy stays alive, we cannot and will not be defeated," said Chan Kin-man, a co-founder of the Occupy Central movement.
The student group Scholarism is planning to lead a boycott of secondary school classes on September 26 to rally further support for the democracy movement, while Occupy is expected to lock down the financial district around October 1.
Hong Kong has been dogged by a series of rallies this summer over the issue of electoral reform. A survey by the Chinese University showed more than a fifth of Hong Kong residents are considering leaving the city, spurred by concerns over its political future.
