By Venus Wu and Hera Poon

HONG KONG (Reuters) - Three Hong Kong student leaders who helped orchestrate the financial hub's massive pro-democracy street protests in 2014 were spared jail time on Monday but ordered to carry out community service.

Teenage activist Joshua Wong, 19, was given 80 hours of community service for unlawful assembly after he and others stormed into a fenced-off area in front of government headquarters called "Civic Square" to stage a sit-in in September 2014.

That sparked a night-long standoff with police and was seen as a key trigger for the "Umbrella Movement" that blocked major roads in the Chinese-ruled city for 79 days in a push for full democracy, presenting Communist Party rulers in Beijing with one of their biggest political challenges in decades.

Two other student leaders were also sentenced.

Alex Chow, 25, was given a three-week prison sentence but granted a reprieve for graduate studies in the United Kingdom.

Nathan Law, 23, who was found guilty of the more serious charge of inciting others to unlawful assembly, was given 120 hours of community service.

"The court believes the three defendants are expressing their views and demands genuinely out of their political beliefs or their concern for society," said district court judge June Cheung.

"Their aim and motive is not for their own interest or to hurt other people."

Hong Kong, a former British colony, was handed back to China in 1997 under an agreement that gave ultimate control to Beijing while promising Hong Kong a high degree of autonomy.

But Beijing's refusal to grant full democracy has exacerbated recent tensions with growing calls for Hong Kong to split from China.

Speaking outside the court besides banners calling for "democracy and self-determination", Chow said the judge's statement carried a "timely warning" for authorities at a time when more people have agitated for independence from China.

"The authorities should consider why so many people are raising these options. What is the motivation, stance and reasons behind them?"

An upcoming legislative election that will be contested by Law and other young activists seeking to enter mainstream politics for the first time is likely to further expose underlying strains in the city of 7.2 million.

"I was really worrying about whether I would be sentenced to imprisonment and that it would affect my election campaign," Law said.

Wong's lawyer, Michael Vidler, said the sentence was fair but that the legal process had been skewed by politics.

"There's a bit of sort of an attitude of ... let's just prosecute everybody despite the weakness of some cases," he said. "They wanted to send a message and very much for Joshua: let's dig up the incidents in the past and see whether we can charge him."

(Reporting by Venus Wu and Hera Poon; Writing by James Pomfret; Editing by Nick Macfie)