The arrest took place on Saturday when riot officers searched demonstrators on a paved area in front of the Consulate where people have camped out in a peaceful human rights protest for close to 60 days. Reports suggest that the teenager was found to have a can of spray paint.

In a statement, the police said that they “responded to an incident at Supreme Court Road reported by a consular staff. Upon police arrival and investigation, one female (aged 15) was arrested outside a consulate in Admiralty for the offence of ‘Criminal Damage’.”

However, human rights groups allege that officers were potentially in breach of diplomatic norms, using site plans to back their claims that the pavement was technically British territory.

“I urge the British government to issue an urgent statement both in defence of the right to peaceful protest and its own boundaries and diplomatic protocols,” said Benedict Rogers, chairman of Hong Kong Watch, urging diplomats to monitor the girl’s case.

“It seems clear from the primary documents that this was inviolable UK land, and that the police should not have been operating there at all, except at the invitation of the Consul General,” added Luke de Pulford, who sites on the Conservative Party Human Rights Commission.

But Eunice Yung Hoi-yan, a barrister and politician with the pro-Beijing New People’s Party, told the South China Morning Post that the police action was legitimate as “the lease specified that the British consulate is the owner of the area, but it has authorised the public to use it.”

When asked to clarify the situation, a Foreign and Commonwealth Spokesperson said: “We are aware of police attending a long running protest outside the Consulate-General in Hong Kong today.”