LONDON - When blind Chinese activist Chen Guangcheng was jailed in 2006, he discovered his fellow inmates spent their days churning out thousands of Christmas lights to be exported to the West.
Chen, who lost his sight as a child after a fever but taught himself law, rose to prominence as a civil rights activist by campaigning for farmers and disabled citizens as well as exposing forced abortions under China's one-child policy.
But in 2006, Chen was jailed on what he said were trumped up charges designed to end his advocacy work. He was kept under house arrest after his release in 2010.
It was while in jail he discovered prisoners were being forced to work from 6am and for up 16 hours a day to turn out products ranging from Christmas ornaments to disposable chopsticks to gift bags.
"Christmas is a festive and jolly season but behind that, there are too many injustices," said Chen, 46, one of China's most high-profile dissidents who lives in the United States.
"These Christmas lights are the products of forced labour," said Chen, now a senior fellow at The Witherspoon Institute, a U.S. think-tank.
Chen hit the headlines in 2012 for triggering a diplomatic row between China and the United States when he escaped house arrest by evading guards at his village home in eastern Shandong province, and fled to the U.S. embassy in Beijing.
Chen took refugee there for days with the spat ending after it was agreed he could go as a student to a U.S. university.
Since then Chen has added the battle against forced labour in China's massive "prison factories" to his list of causes. With daily quotas assigned to them, Chen said inmates often did not have time to go to the bathroom as missing their target could lead to punishments ranging from a beating to being made to stand for hours with their wrists handcuffed to a window.
"Beating happens all the time," said Chen ahead of speaking at the Thomson Reuters Foundation's annual two-day Trust Conference where one day focuses on slavery and trafficking.
"It was going on so much that it was like listening to music in a restaurant - it is always part of the background."
China routinely denies the existence of forced labour and in 2013 abolished a notorious penal system where detainees held without trial carried out such work.
But Chen and human rights campaigners say forced labour remains widespread in China for convicted prisoners under another system known as "reform through labour".
The extent of the use of forced labour in Chinese prisons is not known but the U.S.-China Economic and Security Review Commission, a congressional commission of the U.S. government, said in a report in August that China maintains the practice.
It said products linked to Chinese forced labour included artificial flowers, bricks, Christmas decorations, cotton and footwear but it was hard to be clear on this and stop these exports as often middlemen were used to market the goods.
Chen said he was not forced into manual labour during his imprisonment because he was blind but he urged consumers to think twice before buying cheap goods from China.
An estimated 3.4 million people are living as slaves in China, according to the 2016 Global Slavery Index.
"I would say China's economy is built on the enslavement of the Chinese people," said the lawyer.