The trial of Michael Kovrig, the Canadian detained for more than two years in China on espionage charges, started on Monday, with relations between Ottawa and Beijing in freefall.

The hearing comes days after the closed-door trial of another Canadian man, with both detained in apparent retaliation for Canada's arrest on a US extradition warrant of Huawei executive Meng Wanzhou.

Kovrig, a former diplomat, was detained in 2018 and formally charged last June with spying at the same time as his compatriot, businessman Michael Spavor.

On Monday, police cordoned off an area outside the Beijing court as Canadian diplomats were denied entry and turned away.

Jim Nickel, the charge d'affaires of the Canadian embassy in Beijing, told reporters that he was "very troubled by the lack of access and lack of transparency in the legal process."

Representatives of 26 countries had gathered outside the building on Monday, Nickel said, and were "lending their voice" for Kovrig's immediate release.

A court official told reporters no entry was allowed because the trial is a national security case.

The US is "deeply concerned at the lack of minimum procedural protections granted the two Canadian citizens", William Klein, Acting Deputy Chief of Mission of the US Embassy in Beijing, told media outside the court.

Canadian diplomats were also barred from attending Spavor's trial in the northern city of Dandong on Friday, which lasted less than three hours and ended without any verdict being announced.

Following that closed-door hearing, Canadian Prime Minister Justin Trudeau called the two men's detention "completely unacceptable, as is the lack of transparency around these court proceedings."

China's foreign ministry on Monday defended diplomats being blocked from entering the court, and criticised those gathering outside as "very unreasonable."

"Be it a few or dozens of diplomats trying to gather and exert pressure, it is an interference in China's judicial sovereignty... and not something that a diplomat should do," said foreign ministry spokeswoman Hua Chunying.

'A political case'

The court dates for the two Canadians come as an extradition hearing for Meng enters its final months, and alongside fiery high-level talks between the US and China in Alaska.

Meng, whose father is Huawei founder and CEO Ren Zhengfei, has been fighting extradition to the US on charges that she and the company violated US sanctions on Iran and other laws.

While Kovrig's trial was still ongoing late Monday afternoon, Canada's former ambassador to China, Guy Saint-Jacques, told AFP he expected proceedings will be short.

"China does not even try to make this look like a real trial as evidence is not shared with the defence and the judge does not even take the time to review it," he said ahead of the hearing.

"It just confirms that the process is pre-ordained by the Communist Party and this is a political case."

China's judicial system convicts most people who stand trial and the two men face up to life in prison if found guilty of "espionage" and "providing state secrets".

They have had almost no contact with the outside world since their detention, and virtual consular visits only resumed in October after a nine-month hiatus that authorities said was due to the coronavirus pandemic.

Beijing has insisted the detention of the two Canadians is lawful, while calling Meng's case "a purely political incident".

"The message to the USA is: if you want to help the Canadians, make sure that Meng is returned quickly to China," said Saint-Jacques.