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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.

Read MoreHong Kong braces for protests as China rules out full democracy

"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.

Original post:
Hong Kong students protest for democracy