UPDATE 9-Hong Kong police fire tear gas, water cannon to halt protest as China National Day nears

* Thousands gather downtown near scene of earlier street battles

* Demonstrations mark fifth anniversary of Umbrella movement

* China denies accusations of meddling with Hong Kong freedoms (Adds calm appearing to be restored, paragraphs 8-9)

By Jessie Pang and Donny Kwok

HONG KONG, Sept 28 (Reuters) - Hong Kong police fired tear gas and water cannon on Saturday to disperse anti-government protesters who threw rocks, broke government office windows and blocked a key road near the local headquarters of China's People's Liberation Army (PLA).

A series of protests for and against the city's Communist Party rulers in Beijing is planned ahead of the 70th anniversary of the People's Republic on Tuesday, including at the consulate of former colonial power Britain.

The protesters, many dressed all in black and wearing masks, took cover under umbrellas from the water cannon and fled after the demonstrations had taken a familiar turn into clashes with police as they have done regularly over more than three months.

Police fired blue water, used in other countries to help identify offenders, as protesters regrouped 100 yards (metres) away.

They had smashed windows of government offices and tried to break in, shouting obscenities and daubing slogans on shop-fronts. They also shone lasers at a helicopter hovering overhead.

"A large group of violent protesters are occupying Harcourt Road. They also hurled bricks at police officers. Their acts pose a serious threat to the safety of everyone on site," police said in a statement.

The protesters, some frantically changing out of their tell-tale black clothes behind umbrellas on the way, later headed west towards Central, the main tourism and business district.

The narrow streets between towering office buildings are full of top-brand malls, banks and jewellery shops, and many stores had closed their shutters. But there was no trouble.

There were some loud, angry scuffles between residents and police as arrests were made in the upmarket shopping district of Causeway Bay, to the east of Central, but no major clashes.

Thousands, young and old, had gathered peacefully at a harbourside park to mark the fifth anniversary of the "Umbrella" pro-democracy movement which gridlocked streets for 79 days in 2014. A band was playing Beatles music as the violence erupted on the road next door.

The park sits in front of central government offices and the Legislative Council, both of which have come under attack before, sparking violent street battles with police.

"It’s a special day for Hong Kong protesters. We will stick together to fight for freedom," said Sam, 33, dressed in black and wearing a mask. "Most people think Hong Kong was dying after five years, but many people are still fighting for Hong Kong."