* Little sign of Hong Kong protests flagging
* Nervousness around National Day flag-raising
* Atmosphere on streets resolute, but calm
* Torrential rain makes life uncomfortable for thousands (Updates after National Day flag-raising, paragraphs 1, 9-12)
By Donny Kwok and Irene Jay Liu
HONG KONG, Oct 1 (Reuters) - Thousands of pro-democracy protesters thronged the streets of Hong Kong early on Wednesday, ratcheting up pressure on the pro-Beijing government that has called the action illegal, with both sides marking uneasy National Day celebrations.
There was little sign of momentum flagging on the fifth day of a mass campaign to occupy sections of the city and to express fury at a Chinese decision to limit voters' choices in a 2017 leadership election.
That was despite widespread fears that police may use force to move crowds before the start of celebrations marking the anniversary of the Communist Party's foundation of the People's Republic of China in 1949. The crowds have brought large sections of the Asian financial hub to a standstill, disrupting businesses from banks to jewelers.
Overnight thunder, lightning and heavy rain failed to dampen spirits as protesters sought shelter under covered walkways, while police in raincoats and hats looked on passively nearby. At dawn on Wednesday, protesters awoke to blue skies.
Riot police had used tear gas, pepper spray and baton charges at the weekend to try to quell the unrest but tensions have eased since then as both sides appeared prepared to wait it out, at least for now.
Protests spread from four main areas to Tsim Sha Tsui, one of the city's most popular shopping areas for mainland Chinese, which would normally do roaring trade during the annual National Day holiday.
Underlining nervousness among some activists that provocation on National Day could spark violence, Hong Kong University students made an online appeal not to disturb the flag-raising ceremony that began at 8 a.m. (midnight GMT).
Proceedings went ahead peacefully, although scores of students who ringed the ceremony at Bauhinia Square on the Hong Kong waterfront booed as the national anthem was played.
Hundreds of protesters lined up to view the ceremony, with some organizing a human chain to create a buffer between about 100 police at the site and other demonstrators.
A beaming Hong Kong Chief Executive Leung Chun-ying shook hands with supporters waving the Chinese flag even as protesters who want him to stand down chanted "We want real democracy."
The Hong Kong and Chinese flags billowed in the wind at the completion of the ceremony but one of the main protest groups said they marked the occasion "with a heavy heart."
"We are not celebrating the 65th anniversary of China. With the present political turmoil in Hong Kong and the continued persecution of human rights activists in China, I think today is not a day for celebrations but rather a day of sadness," said Oscar Lai, a spokesman for the student group Scholarism.
Hundreds of demonstrators had gathered outside luxury stores and set up makeshift barricades from the early hours of Wednesday in anticipation of possible clashes. As in most parts of Hong Kong, the police presence was small.
M. Lau, a 56-year-old retiree, said he had taken to the streets of Hong Kong to protest in the 1980s and wanted to do so again in a show of solidarity with a movement that has been led by students as well as more established activists.
"Later this morning I will come back," he said.
"I want to see more. Our parents and grandparents came to Hong Kong for freedom and the rule of law. This (protest) is to maintain our 160-year-old legal system for the next generation."
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, protesters reacted angrily when Beijing decreed on Aug. 31 that it would vet candidates wishing to run for Hong Kong's leadership.
Leung failed to meet an ultimatum from student leaders to come out and address them by midnight on Tuesday but has said Beijing would not back down in the face of protests.
He also said Hong Kong police would be able to maintain security without help from People's Liberation Army (PLA) troops from the mainland.
Communist Party leaders in Beijing 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.
Hong Kong shares fell to a three-month low on Tuesday, registering their biggest monthly fall since May 2012. Markets are closed on Wednesday and Thursday for the holidays.
The city's benchmark index has fallen 7.3 percent over the past month, and there are few indications that the protests are likely to end any time soon. Protesters have set up supply stations with water bottles, fruit, crackers, disposable raincoats, towels, goggles, face masks and tents.
"Even though I may get arrested, I will stay until the last minute," said 16-year-old John Choi. "We are fighting for our future."
MAINLAND CHINESE VISITORS WATCH ON
Mainland Chinese visiting Hong Kong had differing views on the demonstrations, being staged under the "Occupy" banner.
"For the first time in my life I feel close to politics," said a 29-year-old Chinese tourist from Beijing who gave only her surname, Yu. "This is a historic moment for Hong Kong. I believe something like this will happen in China one day."
A woman surnamed Lin, from the southern Chinese city of Shenzhen, however said the protesters' demands for a democratic election were "disrespectful to the mainland."
"Even though the government has brought a lot of development to Hong Kong, they don't acknowledge this," Lin said.
The message from Beijing has been clear.
The deputy director of China's National People's Congress Internal and Judicial Affairs Committee, Li Shenming, wrote in the People's Daily: "In today's China, engaging in an election system of one-man-one-vote is bound to quickly lead to turmoil, unrest and even a situation of civil war."
The outside world has looked on warily.
British finance chief George Osborne urged China to seek peace and said the former colony's prosperity depended on freedom. Washington urged Hong Kong authorities "to exercise restraint and for protesters to express their views peacefully."
U.S. Secretary of State John Kerry will discuss the protests with Chinese Foreign Minister Wang Yi during talks in Washington on Wednesday, U.S. officials said.
The events have also been followed closely in Taiwan, which has full democracy but is considered by Beijing as a renegade province that must one day be reunited with the mainland. (Additional reporting by Farah Master, Diana Chan, Twinnie Siu, Yimou Lee, Kinling Lo, Charlie Zhu, John Ruwitch, Clare Baldwin, Diana Chan and Anne Marie Roantree; Writing by Paul Tait; Editing by Mark Bendeich)