“It is vital that those who are injured are able to receive appropriate medical treatment, and that safe passage is made available for all those who wish to leave the area,” the Foreign Office said.
It called for an end to the violence, “and for all sides to engage in meaningful political dialogue ahead of the District Council elections on Sunday”.
The envoy said the “illegal, violent activities” of protesters had “pushed Hong Kong into an extremely dangerous situation”.
The unrest had eroded China’s “one country, two systems” approach to governing Hong Kong, although he said Beijing remained committed to the policy.
He accused demonstrators of trying to destabilize and paralyze Hong Kong to “seize power” in a push for independence.
“Hong Kong citizens live under black terror created by violent extremism with their life and property under severe threat,” he said.
Hong Kong’s economy has slumped into technical recession because of the months of protests, denting its international image as a place to do business, he said.
“The once ‘Oriental pearl’ is turning into the ‘Oriental scar’. Hong Kong, the ‘fragrant harbor’, is sliding into ‘an abyss of chaos’.
“The future of Hong Kong, if such a situation continues, would be unimaginably dreadful.”