Britain's support for Hong Kong and "its freedoms is unwavering", Foreign Secretary Jeremy Hunt said on Monday, but warned anti-government protesters against violence after they stormed the city's parliament.
The prime ministerial candidate tweeted "to stress UK support for Hong Kong and its freedoms is UNWAVERING on this anniversary day.
"No violence is acceptable but HK people MUST preserve right to peaceful protest exercised within the law, as hundreds of thousands of brave people showed today," he added.
The protesters seized the parliament's main debating chamber late Monday, daubing its walls with graffiti and fixing a British colonial-era flag to the main podium, an AFP reporter on the scene said.
Hunt last week called for an independent investigation into clashes between police and protesters, and suspended export licences for crowd control equipment.
Hong Kong has been rocked by the worst political unrest since its 1997 handover from Britain to China.
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Millions marched last month to oppose a hugely unpopular proposed law that would have allowed extraditions to the Chinese mainland.
Officers then used tear gas and rubber bullets to clear protesters during a massive demonstration outside the city's parliament.