A Cambridge University student who had been detained in a "modern-day hellhole" in South Sudan has been pardoned and released, his lawyer said.

Peter Biar Ajak had been a critic of his country's regime and was studying for a PhD when arrested in July 2018.

He was sentenced to two years in prison in June but his lawyer said he had been pardoned and released on Saturday.

Mr Ajak said "words cannot express how grateful" he was for the support he and his family had received.

"It has meant so much to us and given us the strength we needed. It's a new year and a new decade and my wish is that this year will be the start of lasting peace in South Sudan," he added.

Mr Ajak was studying for a PhD at the time of his arrest

Shortly before his arrest, Mr Ajak had tweeted about South Sudan's "so-called leaders".

His lawyer, Jared Genser, said his client was one of Sudan's "Lost Boys", displaced by the country's civil war.

He resettled in the United States, studying at La Salle University in Philadelphia and Harvard University, before moving to Cambridge University.

Returning to his home country on 28 July 2018 to hold a youth forum, he was arrested and taken directly to custody.

Mr Genser said he was "detained in a modern-day hellhole" for eight months without being charged, before being convicted for having given an interview to a news agency during a prison riot.

But he said Mr Ajak was one of a number of prisoners pardoned by South Sudanese President Salva Kiir on Thursday.

The lawyer said following Mr Ajak's release: "He is a brilliant and tireless advocate for peace in South Sudan and for ensuring the voices of the country's youth are heard and respected. His detention was a travesty of justice."