Student leaders of protests in Hong Kong have said they will step up their actions, including occupying several important government buildings, if the city's leader does not resign by Thursday,

Lester Shum said at a news conference on Wednesday that the leaders would welcome an opportunity to speak with a Chinese central government official, but not chief executive Leung Chun-ying, insisting he must step down.

Shum is the vice secretary of the Hong Kong Federation of Students, which has played a key role in organising the street protests that started Friday.

Tens of thousands of activists, calling for Beijing to drop plans to select candidates for polls in 2017 in the autonomous region, have packed the city centre as China celebrates its National Day.

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Al Jazeera's Fauziah Ibrahim, reporting from the site of the protests, said that even though there has been a festive sentiment since the demonstrations began, there was a feeling of nervousness as crowds gathered on Wednesday.

The activists have occupied several key areas as they press for greater electoral reforms after Beijing decided in August to screen candidates for the territory's first direct election scheduled for two year's time.

President's remarks

In a speech on Tuesday, President Xi Jinping did not make any direct reference to the ongoing protests, but pledged that China would protect Hong Kong's interests.

"The central government will unswervingly implement the guidelines of 'one country, two systems' and the Basic Law and steadfastly safeguard the long-term prosperity and stability of Hong Kong and Macau," Xi said. 

The protests are the worst in Hong Kong since China resumed its rule of the former British colony in 1997.

They also represent one of the biggest political challenges for Beijing since it violently crushed pro-democracy protests in Tiananmen Square in 1989 .

Cracking down too hard could shake confidence in market-driven Hong Kong, which has a separate legal system from the rest of China.

Not reacting firmly enough, however, could embolden dissidents on the mainland.

China rules Hong Kong under a " one country, two systems " formula that accords the former British colony a degree of autonomy and freedoms not enjoyed in mainland China, with universal suffrage set as an eventual goal.

However, when Beijing decreed a month ago that it would vet candidates wishing to run for Hong Kong's leadership, protesters reacted angrily.

Source: Al Jazeera and agencies