A top-secret document circulating among senior police and security chiefs in the Democratic Republic of Congo suggests that individuals deported from Britain may face torture on their return to their home country.

Details of the document have emerged as the Home Office begins to detain dozens of Congolese nationals for removal, despite concerns over human rights abuses.

The highly sensitive document is an order from Congo's ministry of the interior, circulated to senior officials in the national intelligence agency, Agence Nationale de Renseignements, the police and the Direction Générale de Migration and leaked to the Observer. It instructs security chiefs to track down and arrest opponents of the government, including members of the main opposition party, the Union for Democracy & Social Progress, and suggests torture could be used with "discretion".

Emphasis is placed on targeting political activists living in the UK and other parts of Europe who are forcibly removed to the Congolese capital, Kinshasa. They are referred to as "combatants", or traitors considered to be fighting against the government.

Written in French, the document states: "Above all, be on the lookout for the return to the country (of combatants) by refoulement" (a term referring to the expulsion of persons who have the right to be recognised as refugees).

Officers are asked to "intensify the rigour" with which they uncover these combatants. It adds: "A list and certain photos will be sent to you without delay."

The document then states: "The treatment reserved for these people is clear: torture and other things must be done with the greatest discretion. These orders must be carried out flawlessly."

Human rights organisations have frequently raised concerns over the country's human rights abuses. A Home Office fact-finding mission, published in November 2012, documented eight human rights organisations in the DRC who said failed asylum seekers were at risk of torture on their return.

A report by Justice First monitored 11 returnees to the DRC from the UK for a period up to September 2013. Nine had been detained, of whom eight were imprisoned and two died, one apparently after receiving severe beatings from Congolese officials.

The document passed to the Observer urges police, immigration and secret service officials to be on the lookout for asylum seekers who have been forcibly returned at the different border posts in Kinshasa, including Ndjili airport, Ngobila Beach on the border between Congo and Congo Brazzaville, and other secret entry points along the river which borders the two countries.

The apparent confirmation that returnees from the UK could be tortured has emerged as the British government prepares to start removing failed asylum seekers to the DRC after a two-year period in which only a handful have been deported due to concerns about the risk of torture.

The Observer has learned that failed Congolese asylum seekers throughout the UK have recently been arrested and detained in various immigration detention centres. One law firm has identified 13 recent arrests, while other lawyers have confirmed more cases.

One Congolese asylum seeker who is five months pregnant is being held at Yarl's Wood Immigration Detention Centre in Bedfordshire. She said: "I won't survive if they send me back. I was arrested last week when I went to sign on in Cardiff. Another young pregnant Congolese woman arrived here this week. There are six women from Congo here altogether and we are all very scared about what will happen to us if the British government sends us back home."

A Home Office spokesman said: "The UK has a proud history of granting asylum to those who need it and we consider every application on its individual merits.

"Any decision to refuse asylum is made on the basis that it is safe for someone to return to their country of origin. The courts have ruled that failed asylum seekers who are returned to the Democratic Republic of Congo (DRC) are not at risk of treatment contrary to article three of the European Convention on Human Rights."