Two British human rights activists have "disappeared" while investigating the treatment of Qatar's migrant laborers.

Krishna Upadhyaya, 52, and Ghimire Gundev, 36, vanished on the evening of Saturday August 30, according to their employer, after Uphadhyaya sent an urgent text to a friend in Norway saying: "I am being followed by the police here. Looks like they will give me troubles now."

The men work for the non-governmental organisation Global Network for Rights and Development (GNRD), which has not heard from them for more than four days and fears they have been detained.

On Friday afternoon around 20 protesters, including representatives from Amnesty International, Prospect and Anti-Slavery International, assembled outside Qatar's embassy in London to hand a letter to Qatari authorities demanding news on Upadhyaya and Gundhev's whereabouts.

The letter, which was addressed to the ambassador of Qatar in the UK, criticised the actions of Qatar's police and said: "We demand that the government of Qatar discloses what it is doing to find the two workers and ensure their safety."

Jakub Sobik, a colleague of Upadhyaya at Anti-Slavery International, where he worked for 11 years before leaving in 2013, said the embassy had so far failed to respond to requests for news of the pair.

He told VICE News: "[Upadhyaya] was a good colleague, always committed to the cause with a very strong sense of justice and that was what was driving him on.

"We're demonstrating and handing a letter over to the Qatari authorities to ensure that they disclose what they're doing and ensure that Krishna is found safe."

The men arrived in Qatar on August 27 to conduct research for a report on the conditions under which its Nepalese migrant workforce labors. The country has repeatedly drawn criticism from human rights groups as it oversees an ambitious construction effort in preparation for the 2022 World Cup.

In May, a report commissioned by Qatar revealed that 964 migrant workers from Nepal, Bangladesh and India had died on its soil in 2012 and 2013, many from unexplained sudden illness, according to the government's own figures. The study, by international law firm DLA Piper called for changes to the much-criticised kafala system under which migrant workers are tied to their sponsoring employers. Human rights groups such as Amnesty International say that 90 percent of such workers have their passports taken away and suffer widespread abuse and exploitation.

Following the report, Qatar promised to replace the sponsorship system with one of contracts, but activists criticised the proposal as little more than a fig leaf amounting to a change of name rather than genuine reform.

The last contact the GNRD had with its researchers was on August 31, when the men prepared to leave their hotel for the airport. Upadhyaya had expressed concern about police in the area and noticed undercover operatives close to him as he used the phone, according to GNRD.

In a statement GNRD said it was "deeply concerned" about the welfare of the researchers and was considering legal action.

The statement said: "It is really regrettable that Qatar, which is signatory state to numerous international law conventions, especially on Enforced Disappearances, is carrying out such explicit violations of human rights."

Sarita Poudyal, the wife of Upadhyaya, wrote on Facebook: "The Qatari government must release them as soon as possible in safe conditions."

In a statement Gundev's family also called on the Qatari authorities to release the men, describing the Gundev, a photographer, as a "kind-hearted man who is always interested in helping the disabled and poor."

Neil Durkin, a spokesperson for Amnesty International UK, said the case was "ominous" and unprecedented.

"In the past there have been one or two issues with journalists being detained, but only temporarily and nothing as serious as this, so it does seem to be a departure from what's gone before," he told VICE News.

"There are numerous abuses that sprang from the kafala system used to monitor the migrant labourers in Qatar, including people not being paid for many months, living in squalid conditions, and being compelled to work for long hours in difficult and dangerous conditions.

"There have been the beginnings of reform but they haven't gone very far yet and we're worried that the Qatari authorities aren't taking this as seriously as they should be. There's still a long way to go."

A spokesperson for the UK Foreign Office said: "We are aware of two British nationals who have been detained by the Qatari authorities. We have requested urgent consular access."