Hong Kong Watch has condemned the "absurdly excessive use of force" by police in the city following an escalation in violence in recent days.
The pro-democracy campaign group made an urgent appeal for peace in the former British colony after clashes between police and protesters on Monday led to an officer firing a live round at a young man, while another attempted to run down protesters using a motorbike.
The campaign group said that the Hong Kong police "must not be allowed to act with impunity".
"Their actions are a clear breach of the rights and freedoms enshrined in the handover agreement, curtailing freedom of expression, freedom of association, and potentially in the case of the man now in a critical condition: the right to life," it said.
It went on to condemn violence perpetrated by the pro-democracy protesters after a pro-Beijing supporter was doused in petrol and set alight later in the day.
"Hong Kong Watch condemns this and calls for the deescalation of violence," it said.
Last Friday, a young man died from injuries sustained while trying to flee from tear gas. Then on Saturday, the South China Morning Post reported that a girl had received an abortion after allegedly being gang raped in a Hong Kong police station.
Hong Kong Watch called the events "disturbing" and has demanded an urgent investigation into the police by an independent body.
"An independent panel of experts have concluded that the Independent Police Complaints Council are unable to adequately investigate," the group said.
"Hong Kong Watch urges the Hong Kong Government to establish an independent inquiry into police brutality."
It called on the UK to "consider every available means" to uphold the Sino-British Joint Declaration underpinning the 'one country, two systems' principle that came into effect after the 1997 handover.
"Hong Kong is at a critical moment and it is vital that the international community stands with the people of Hong Kong," it said.
It continued: "The United Kingdom government, as a co-signatory of the Sino-British Joint Declaration, should consider every available means of action to stand in solidarity, including the extension of the rights of British National (Overseas) passport holders, the enactment of Magnitsky sanctions against the perpetrators of human rights violations, and the creation of a UN contact group to coordinate diplomacy with like-minded nations.
"The United States Congress should pass the Hong Kong Human Rights and Democracy Act."
Benedict Rogers, founder of Hong Kong Watch and East Asia Team Leader for Christian Solidarity Worldwide, said on Twitter: "Hong Kong's crisis is heartbreaking. Armistice Day has become the Darkest Day ... The world must now act to stop this barbaric carnage."