LONDON, Sept 29 (Reuters) - Britain's Foreign Office said on Monday it was concerned about the situation in Hong Kong, which is facing the worst unrest since China resumed its rule of the former British colony in 1997, and called for the right to protest to be protected.

"It is Britain's longstanding position, as a co-signatory of the Sino-British Joint Declaration, that Hong Kong's prosperity and security are underpinned by its fundamental rights and freedoms, including the right to demonstrate," the foreign office said in a statement.

"It is important for Hong Kong to preserve these rights and for Hong Kong people to exercise them within the law." (Reporting by Michael Holden; editing by Stephen Addison)