Protest leaders clarified their ultimatum for the Hong Kong government on Wednesday, after pro-democracy demonstrators turned their backs on a Chinese flag-raising ceremony on National Day.
"Because the government ordered police to fire 87 rounds of tear gas at protests, there is now no room for dialogue. Chief executive Leung Chun-ying must step down," said Lester Shum, vice secretary of the Hong Kong Federation of Students.
"If he doesn't resign by (Thursday) we will step up our actions, such as by occupying several important government buildings," Shum said.
But the organisers backtracked on an earlier threat to widen the protest areas or stage a labour strike.
The activists are demanding reforms ahead of the 2017 elections, including open nominations of candidates for next chief executive.
Visitors from mainland China gawked at Wednesday's big crowd of pro-democracy protesters. One mainlander shouted, "Go home and stop inconveniencing people."
Fireworks planned for the evening to mark National Day were cancelled, city officials said.
Student leader Joshua Wong, 17, said the cancellation of holiday celebrations showed the government was under heavy pressure.
Wong reiterated protesters' demands for Beijing to withdraw its election reform plan and for Hong Kong's current leader to resign.
Beijing has been left grappling with one of the biggest challenges to its rule over the city at a time when the Communist Party is cracking down hard on dissent on the mainland.
The most intense civil unrest Hong Kong has experienced since its 1997 handover from British rule was sparked by Beijing's decision in August to restrict who can stand for the city's top post.
Hong Kongers will be able to vote for their next chief executive in 2017 but only two or three candidates vetted by a pro-Beijing committee will be allowed to stand.
Protesters call this fake democracy. They have two demands - that Leung step down and that Beijing rescind its insistence that candidates for future leaders be vetted.
But Beijing has stayed defiant, saying it supports Hong Kong's handling of the protests. Analysts say the chance of the Chinese government backing down is virtually non-existent.
In comments to mark the National Day anniversary, Chinese President Xi Jinping said "all tumours grown on the healthy organism" of the Communist Party must be removed.
"We must never waver in our faith and must never separate ourselves from the people," Xi added at the commemoration, according to Xinhua news agency.
