* Tuesday marks 70th anniversary of People's Republic of China
* Protests planned across Hong Kong; malls, metro stations closed
* Hong Kong police warn of "very serious violent attack"
* Acting leader promises more dialogue to ease tensions (Adds acting leader's quotes, colour on preparations)
By Anne Marie Roantree and Farah Master
HONG KONG, Oct 1 (Reuters) - Hong Kong went into lockdown on Tuesday, with barricades in the city centre, shuttered stores and a heavy riot police presence, as authorities scrambled to ensure anti-government protests do not overshadow China's anniversary festivities in Beijing.
The former British colony has been wracked by nearly four months of street clashes and demonstrations, posing the gravest popular challenge to Chinese President Xi Jinping since he came to power.
Protesters have vowed to seize on the holiday, marking the 70th anniversary of the founding of the People's Republic of China, to propel their calls for greater democracy onto the international stage, hijacking an occasion Beijing sees as an opportunity to showcase China's economic progress.
Police said on Monday they expected a "very serious violent attack". Authorities have rejected a permit submitted by protesters for a planned march, but demonstrations are expected across the city regardless.
Hundreds of officials and the pro-establishment elite began the day with a formal flag-raising ceremony and National Day reception at the Convention and Exhibition Centre, held early and moved behind closed doors. Roads leading to the centre were closed and tightly policed.
"Under 'one country, two systems', Hong Kong has benefitted from China's support," Acting Chief Executive Matthew Cheung told the assembly, referring to guarantees of political freedoms for the territory after its handover from British to Chinese rule in 1997.
"Some radical protesters' ongoing and escalating protests ... unlawfully blocking roads, throwing petrol bombs ... affect the safety and normal life of citizens, and worsen a declining economy," he added.
Cheung called a first "open dialogue" held last week with citizens an important step, and said more would follow.
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A group of protesters outside the venue shouted "No national day celebrations, only national day mourning", and called for those arrested during recent clashes to be released. Police fired pepper spray during a scuffle outside a nearby station.
Rail operator MTR Corp closed some flash-point metro stations, and several large shopping malls said they would close to avoid damage in any clashes.
The government of embattled leader Carrie Lam has already cancelled the annual Oct. 1 fireworks display over the city's iconic Victoria Harbour, citing public safety concerns.
Lam, who was trapped in an indoor stadium for hours last week after attending the "open dialogue", left for Beijing on Monday to celebrate China's birthday on the mainland. She will return on Tuesday night.
In contrast to events in Hong Kong, Beijing's carefully choreographed anniversary festivities are expected to involve troops marching through part of Tiananmen Square with new missiles and floats celebrating the country's technological prowess.
The Communist Party leadership is determined to project an image of national strength and unity in the face of challenges including Hong Kong's political unrest, slowing economic growth and a trade war with the United States.
Hong Kong protesters are angry about what they see as creeping Chinese interference in the Asian financial centre, which Britain returned to China under a formula guaranteeing freedoms that are not enjoyed on the mainland.
China dismisses the accusation and has accused foreign governments, including the United States and Britain, of fanning anti-China sentiment.
Last month, Beijing moved thousands of troops across the border into the restive city, a move state news agency Xinhua described as a routine "rotation". Asian and Western envoys in Hong Kong, however, have said the absence of any evidence that troops had been withdrawn suggested it was a reinforcement, with the largest-ever regular army force now stationed in the city.
On Sunday, Hong Kong police fired water cannon, rubber bullets and tear gas at petrol bomb-throwing protesters in some of the most widespread and violent clashes in months of anti-government unrest. (Reporting by Anne Marie Roantree and Farah Master, Additional reporting by Jessie Pang; Editing by Stephen Coates)
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