HONG KONG (Reuters) - Tens of thousands of pro-democracy protesters blocked Hong Kong streets in the early hours on Tuesday, maintaining pressure on China as it faces one of its biggest political challenges since the Tiananmen Square crackdown 25 years ago.

Riot police had largely withdrawn and there were none of the clashes, tear gas and baton charges that had erupted over the weekend. As tensions eased, some exhausted demonstrators slept on roadsides while others sang songs or chanted slogans.

One young police officer relaxed in a chair and played on his mobile phone as thousands of demonstrators milled in the streets nearby, some singing and dancing.

Asked why there were so few police, he replied: “Actually, I don’t have a reason for you. But we are tired. We are all human beings so we need a rest.”

The protesters, mostly students, are demanding full democracy and have called on the city’s leader Leung Chun-ying to step down after Beijing last month announced a plan to limit 2017 elections for Hong Kong’s leader, known as the Chief Executive, to a handful of candidates loyal to Beijing.

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.

Communist Party leaders worry that calls for democracy could spread to the mainland, and have been aggressively censoring news and social media comments about the Hong Kong demonstrations.

The outside world has looked on warily, concerned that the clashes could spread and trigger a much harsher crackdown.

“The United States urges the Hong Kong authorities to exercise restraint and for protesters to express their views peacefully,” White House spokesman Josh Earnest told a daily briefing on Monday.

The demonstrations, labeled “illegal” by China’s Communist-run government in Beijing, are the worst in Hong Kong since China resumed its rule over the territory in 1997.

At their height, white clouds of tear gas wafted among some of the world’s most valuable office towers and shopping malls, before riot police suddenly withdrew around lunchtime on Monday.

As tensions subsided, weary protesters dozed or sheltered from the sun beneath umbrellas, which have become a symbol of what some are calling the “umbrella revolution”.

In addition to protection from the elements, umbrellas have been used as flimsy shields against pepper spray.

Organizers said that as many as 80,000 people thronged the streets after the protests flared up on Friday night. No independent estimate of numbers was available.

On Monday and early Tuesday, protesters massed in at least four of Hong Kong’s busiest areas, including Admiralty, where Hong Kong’s government is headquartered, the Central business district, Causeway Bay, known for its shopping, and the densely populated Mong Kok district in Kowloon.

“I must stress that the events happening now cannot be attributed to the students or Occupy Central. It has evolved into a civil movement,” said leader of the Hong Kong Federation of Students, Alex Chow.

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INTERNATIONAL CONCERN

The movement puts Beijing’s ruling Communist Party in a difficult position. Cracking down too hard could shake confidence in market-driven Hong Kong, while not reacting firmly enough could embolden dissidents on the mainland.

The protests are expected to escalate on Oct. 1, China’s National Day holiday, with residents of the nearby former Portuguese enclave of Macau planning a rally.

Pro-democracy supporters from other countries are also expected to protest, potentially causing further embarrassment.

Televised scenes of the chaos in Hong Kong over the weekend have already made a deep impression outside the financial hub.

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That was especially the case in Taiwan, which has full democracy but is considered by China as a renegade province that must one day be reunited with the Communist-run mainland.

Taiwan President Ma Ying-jeou said Beijing needed “to listen carefully to the demands of the Hong Kong people”.

Britain said it was concerned about the situation and called for the right of protest to be protected.

Earlier, Chinese Foreign Ministry spokeswoman Hua Chunying said Beijing was “resolutely opposed to any country attempting in any way to support such illegal activities like ‘Occupy Central’.”

“We are fully confident in the long-term prosperity and stability of Hong Kong, because I believe this is in keeping with the interests of all the people in China, the region and the world,” she said.

In 1989, Beijing’s Tiananmen crackdown sent shockwaves through Hong Kong as people saw how far China’s rulers would go to keep their grip on power.

SOME BANKS PULL DOWN SHUTTERS

Banks in Hong Kong, including HSBC HSBA.L, Citigroup C.N, Bank of China 601988.SS, Standard Chartered STAN.L and DBS DBSM.SI, shut some branches and advised staff to work from home or go to secondary branches.

While the financial fallout from the turmoil has been limited so far, Hong Kong shares .HSI ended down 1.9 percent on Monday.

About 200 workers at Swire Beverage, a unit of Hong Kong conglomerate Swire Pacific 0019.HK and a major bottler for the Coca-Cola Company KO.N, went on strike in support of the protesters, a union representative said. They also demanded the city's leader step down.

The protests have spooked tourists, with arrivals from China down sharply ahead of this week’s National Day holidays. Hong Kong on Monday canceled the city’s fireworks display over the harbor, meant to mark the holiday. The United States, Australia and Singapore issued travel alerts.

In Kowloon, across the harbor from Central district, tens of thousands of people packed the streets with no police in sight. The protesters were highly organized, with supply stations stacked with water bottles, fruit, biscuits, chocolate bars and other food.