In the letter revealed on Monday, the four-member UN group expressed concern over allegations that volunteer health care workers had been intimidated during the months of social unrest.

Among the cases cited was that of at least 16 volunteer first-aiders detained by police during the siege of Polytechnic University while wearing vests identifying themselves as doctor, nurse, or “emergency medical technician”.

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The UN group is comprised of special rapporteurs Dainius Puras, Clement Voule and Joseph Cannataci as well as Leigh Toomey, a vice-chair of a working group on arbitrary detention.

Another case cited from the experts’ own research involved a man who was “zip tied” by police for several hours in August despite complaining of “intense pain”.

“He suffered multiple fractures, and doctors indicated that the nature of the injuries was extremely serious and clearly the result of severe and intense beating,” they wrote.

“[The] acts described above appear to contravene Article 3 of the Universal Declaration of Human rights which guarantee the rights of every individual to life, liberty and security.”

They stressed that the International Covenant on Civil and Political Rights (ICCPR), to which Hong Kong is a signatory, also underlined a duty to protect life, including “ensuring access without delay to essential health care”.

In their letter, the group asked both the Beijing and local Hong Kong governments to explain what measures were taken to quickly reach injured persons during protests, as well as the legal basis for arresting or detaining health care workers.

The letter from the rapporteurs, who are appointed by the UN Human Rights Council, came as a group of British lawmakers planned to launch their own inquiry into police handling of protests and whether human rights violations had occurred.

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The bipartisan All-Party Parliamentary Groups, comprised of members from the House of Commons and House of Lords, had received nearly 1,000 submissions as of their Monday deadline, according to its secretariat.

A submission made to the group by Civil Rights Observer, a Hong Kong NGO, cited six volunteer first-aiders who complained about the use of tear gas and the fact police had stopped them from treating injured protesters.

“The Hong Kong Police Force indiscriminately attacked these humanitarian supporters, or deterred them from providing help to injured persons,” said Icarus Wong Ho-yin, founder of the group.

A spokesman for the Constitutional and Mainland Affairs Bureau (CMAB) said: “In response to a letter from the United Nations (UN) Special Rapporteurs to the Ministry of Foreign Affairs of the People’s Republic of China, the Government of the Hong Kong Special Administrative Region has provided response to the Central People’s Government for onwards transmission to the United Nations.”

The Hong Kong Police Force said it had nothing to add beyond the reply given by the CMAB.

The Chinese mission to the UN has previously expressed support for the police force and rejected criticisms over their handling of the protests.

“Administrative matters in Hong Kong are purely the internal affairs of China. No country, organisation or individual has the right to interfere in them,” the mission said in September.