HONG KONG (Reuters) - Hong Kong police fired water cannon and volleys of tear gas to disperse protesters throwing petrol bombs at government buildings on Sunday, as months of sometimes violent demonstrations showed no sign of letting up.

Some protesters threw bricks at police outside the Chinese People’s Liberation Army base in the city’s Admiralty district, and tore down and set fire to a red banner proclaiming the 70th anniversary on Oct. 1 of the founding of the People’s Republic of China, in a direct challenge to Beijing.

One water cannon caught fire after being hit by a petrol bomb. The water cannon fired blue jets of water, used elsewhere in the world to help identify protesters later.

“Radical protesters are currently occupying Harcourt Road in Admiralty, vandalizing Central Government Offices and repeatedly throwing petrol bombs inside,” police said in a statement.

Thousands of protesters, many clad in black masks, caps and shades to obscure their identity, raced through the streets of the financial hub in cat-and-mouse tactics with police, setting street fires and blocking roads in the heart of the city.

Authorities moved quickly to douse the fires and police fired volleys of tear gas to disperse protesters, including in the bustling shopping and tourist district of Causeway Bay.

Violence erupted in the district of Fortress Hill on the east of the island as men in white T-shirts, some wielding rods, clashed with anti-government activists.

A heavy police presence could be seen in and around subway stations. Rail operator MTR Corp has become a prime target of vandalism, with activists angry that it closes stations during protests and prevents demonstrators from gathering.

Shops in key protest areas once again shuttered early as more than three months of demonstrations continued to take a toll on business.

The Airport Authority said on Sunday passenger numbers fell 12.4% year-on-year in August to six million. Protesters last month jammed the airport arrivals hall, leading to canceled or delayed flights as they sought to draw world attention to their fight for democracy.

But while chaotic scenes of protesters clashing with police have been beamed live to the world - at times under gleaming skyscrapers in the heart of the financial center - life for many in the Chinese-ruled territory proceeds relatively normally.

BRITISH CONSULATE RALLY

While the turnout on Sunday was smaller than previous weekends, the unrest underscores the defiance of many activists.

Demonstrators are angry about what they see as creeping interference by Beijing in their city’s affairs despite a promise of autonomy.

The spark for the protests was planned legislation, now withdrawn, that would have allowed people to be sent to mainland China for trial.

The protests have since broadened into calls for universal suffrage.

Earlier on Sunday, protesters gathered peacefully outside the British Consulate, calling on Britain to rein in China and ensure it respects the city’s freedoms.

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The Sino-British Joint Declaration, signed in 1984, lays out Hong Kong’s future after its return to China in 1997, a “one country, two systems” formula that ensures freedoms not enjoyed on the mainland.

“Sino-British Joint Declaration is VOID,” one placard read in the protest outside the British Consulate.

“SOS Hong Kong,” read another.

“One country, two systems is dead,” protesters shouted in English under umbrellas shielding them from the sub-tropical sun, some carrying the colonial flag also bearing the Union Jack. “Free Hong Kong.”

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China says it is committed to the arrangement, denies meddling and says the city is an internal Chinese issue. It has accused foreign powers, particularly the United States and Britain, of fomenting the unrest and told them to mind their own business.

Britain says it has a legal responsibility to ensure China abides by the 1984 declaration.

Hong Kong island was granted to Britain “in perpetuity” in 1842 at the end of the First Opium War. Kowloon, a peninsula on the mainland opposite Hong Kong island, joined later, after the Second Opium War.

The colony was expanded to include the New Territories, to the north of Kowloon, on a 99-year lease, in 1898.

Britain returned all of the territory to China, which never recognized the “unequal treaties”, in 1997.

“The Joint Declaration is a legally binding treaty between the UK and China that remains as valid today as it was when it was signed and ratified over 30 years ago,” a British Foreign Office spokeswoman said in June.

“As a co-signatory, the UK government will continue to defend our position.”

But it was not immediately clear what Britain could or would want to do to defend that position.

It is pinning its hopes on closer trade and investment cooperation with China, which since 1997 has risen to become the world’s second-largest economy, after it leaves the European Union at the end of next month.