Chinese authorities have carried out their first executions of Tibetans in connection with the deadly riots that swept Lhasa last year, according to exile groups.

As the first reported judicial killings in the region for six years, the news has prompted overseas protests and concerns that proper legal procedures were not followed.

The Chinese state media have yet to confirm the executions. However, the Tibetan Centre for Human Rights and Democracy, based in Dharamsala in northern India, said it had reports that they took place early on Tuesday morning.

It identified three of the executed Tibetans as two men – Lobsang Gyaltsen and Loyak – and a woman named Penkyi. The fourth victim was not named.

The Lhasa intermediate people's court sentenced the two men to death on charges of arson in April, according to Xinhua news agency reports at the time. Under Chinese law, death penalty cases should be reviewed by the supreme court.

The Dharamsala-based group said the body of Lobsang Gyaltsen had been handed to his family, while Loyak's ashes were given to his relatives.

The director of Free Tibet, Stephanie Brigden, said the executions were an outrage. "It is impossible to have any confidence that even the most basic legal norms were observed before the Chinese state sanctioned and carried out the killing of these four Tibetans," she said.

London-based Tibetan groups called for a vigil outside the Chinese embassy. Free Tibet said the executions, which came just weeks after a Foreign Office minister, Ivan Lewis, made a rare trip to Lhasa, should prompt the British government to rethink the way it engages with China over the region.

Labour MP Kate Hoey has tabled an early day motion calling for an inquiry into the UK government's failure to secure human rights improvements in Tibet.

A US congressional commission will publish a study of Tibet next week which finds Chinese officials are strengthening efforts to separate Tibetan Buddhists from the Dalai Lama, and preparing to select his successor. However the study also notes that the Dalai Lama's willingness to restrict discussions only to areas that China considers part of Tibet have opened up an unprecedented opportunity for progress.

In a separate case, Reporters Without Borders called for the release of three Tibetans who have been held since 1 October for allegedly sending information about Tibet to contacts abroad via the internet.

It said the three have not been permitted to contact their families during detentions.

"The internet is monitored, censored and manipulated more in Tibet than in other Chinese provinces," Reporters Without Borders said. "Despite the risks, Tibetan internet users continue to transmit information, especially to the diaspora and human rights groups. It is deplorable that the Chinese police devote so much energy to identifying and arresting ordinary internet users."