* First legal pro-democracy rally since emergency laws

* Tens of thousands call for U.S. help

* Police say violent protests now 'life-threatening'

* Homemade bomb detonated near police

* President Xi says he would crush attempts to split China (Updates with major rally, protesters' quotes)

By John Ruwitch and Jessie Pang

HONG KONG, Oct 14 (Reuters) - Tens of thousands of mostly young pro-democracy activists rallied in Hong Kong on Monday in the first legal protest since the introduction of colonial-era emergency laws and pleaded for help from the United States.

They chanted "Fight for Freedom, Fight for Hong Kong" as they gathered peacefully near central government offices in the Admiralty district of the Chinese-ruled city only hours after police said violent protests had escalated to a "life-threatening level".

A small bomb exploded and a policeman was stabbed on Sunday night, the latest violence in four months of unrest in which police have responded to petrol bombs and rocks with tear gas, rubber bullets, water cannon and sometimes live rounds.

Emergency laws introduced on Oct. 5 banning face masks at rallies and carrying a maximum penalty of one year in jail sparked some of the worst violence since the unrest started.

On Monday night, many protesters wore face masks in defiance of the ban.

Speakers urged the United States to pass a Hong Kong human rights act to ensure democracy for the former British colony, which returned to Chinese rule in 1997.

"Make Hong Kong Great Again", read one poster. Some protesters waved the U.S. flag and carried "Uncle Sam" recruitment posters reading "Fight for Freedom, Stand with HK".

"All of the Hong Kong people feel hopeless and the government hasn't listened to our voices so we need the USA to help us," said protester Edward Fong, 28.

The protesters are angry at what they see as Beijing's tightening grip on the city which was guaranteed 50 years of freedoms under the "one country, two systems" formula under which it returned to China. Beijing rejects the charge and accuses Western countries, especially the United States and Britain, of stirring up trouble.

The unrest poses the biggest popular challenge to Chinese President Xi Jinping since he came to power in 2012. He warned that any attempt to divide China would be crushed.

"Anyone attempting to split China in any part of the country will end in crushed bodies and shattered bones," Xi said in a meeting on Sunday with leaders in Nepal, where he was visiting, according to China's state broadcaster CCTV.

'THEY ARE RIOTERS, CRIMINALS'

In contrast to Monday night's peaceful protest, rallies descended into chaos on Sunday with running skirmishes between protesters and police in shopping malls and on the streets.

Black-clad activists threw 20 petrol bombs at one police station, while others trashed shops and metro stations.

A crude explosive device, which police said was similar to those used in "terrorist attacks", was remotely detonated as a police car drove past and officers were clearing roadblocks on Sunday night.

A police officer also had his neck slashed by a protester.

"Violence against police has reached a life-threatening level," said Deputy Commissioner of Police Tang Ping-keung.

"They are not protesters, they are rioters and criminals. Whatever cause they are fighting for it never justifies such violence."

Protests have attracted millions of people but have gradually become smaller in recent weeks. Yet violence by hardcore activists has risen, prompting debate over tactics. But they say they remained united.

"Violence is always undesirable, but in the case of Hong Kong, we have no other option," said regular protester Jackson Chan, 21.

"In June, 2 million took to the street and demonstrated peacefully, yet the government showed a complete disregard to the public opinion... Escalation of violence is inevitable," Chan said.

On Monday, speakers called on U.S. senators to vote for the Hong Kong Human Rights and Democracy Act of 2019, saying it would be their "most powerful weapon".

The bill supports human rights in Hong Kong with measures under consideration such as annual reviews of its special economic status and sanctions on those who undermine its autonomy. The text will not be finalised until it passes both houses of Congress and is signed by the president.

"We are exhausted and scared, many of us have been detained and tortured... We believe international help will come one day," said one speaker.

Police have fired thousands of rounds of tear gas and rubber bullets at brick- and petrol bomb-throwing protesters and arrested more than 2,300 people since June, many teenagers. Two people have been shot and wounded.

Hong Kong leader Carrie Lam is due to deliver her annual Policy Address on Wednesday amid pressure to restore confidence in the government.

Hong Kong is facing its first recession in a decade because of the protests, with tourism and retail hardest hit. (Additional reporting by Anne Marie Roantree and Donny Kwok in Hong Kong Writing by Michael Perry Editing by Robert Birsel and Nick Macfie)

Our Standards: The Thomson Reuters Trust Principles.