A former Canadian diplomat detained in China last week is being denied legal representation and is not allowed to turn lights off at night, sources say.
Michael Kovrig, who now works for the International Crisis Group (ICG) think tank, was detained on accusations of harming national security.
Canada drew Chinese protests after it arrested an executive at telecoms giant Huawei at the request of the US.
China says it has guaranteed Mr Kovrig's lawful rights.
ICG spokesman Karim Lebhour told the BBC: "Michael has not been allowed access to his lawyers. The arrest is unjustified."
Mr Kovrig has been working as North East Asia senior adviser for the Brussels-based think tank since February 2017.
Meanwhile an unnamed source told Reuters news agency that he was being held at an undisclosed location and being questioned every morning, afternoon and evening.
He is not allowed to turn off the lights when he tries to sleep at night, the person added.
"He is physically all right, but tired and stressed," a source told the Financial Times. "Physically, he does not appear mistreated."
China denied Mr Kovrig was being mistreated.
"We have already said that China has in accordance with the law guaranteed Michael Kovrig's lawful rights and humanitarian treatment, and has provided Canada with necessary help to carry out normal consular work," foreign ministry spokeswoman Hua Chunying told a press briefing, referring questions to the "relevant authorities".
Canada only gained consular access to Mr Kovrig at a police station on 14 December, when he was visited for half an hour by the Canadian ambassador and two other Canadian diplomats, the sources said.
On Friday Foreign Minister Chrystia Freeland said Canada was "deeply concerned by the arbitrary detention" of the two men and called for their immediate release.
Mr Kovrig was arrested in Beijing on 10 December, they added.
A second Canadian - Michael Spavor, a businessman - was also detained last week and is facing the same accusations.
Canada arrested Huawei chief financial officer Meng Wanzhou earlier this month, at the request of the US, which is engaged in a trade war with China.
Ms Meng faces extradition to the US to face fraud charges linked to allegations of avoiding US sanctions on Iran. Each charge carries a maximum possible sentence of 30 years jail.
China has denied the detention of both men is tied to Ms Meng's arrest, but many analysts believe it was a tit-for-tat action.