A group of Hong Kong pro-democracy activists who tried to escape the city by speedboat for sanctuary in Taiwan went on trial in China on Monday as the United States urged the immediate release of dissidents it says fled tyranny.

Ten of the so-called Hong Kong 12 were in court in the southern city of Shenzhen to face charges linked to an illegal border crossing.

Chinese authorities took them into custody after their boat was intercepted on August 23.

The hearing began as scheduled on Monday afternoon and a convoy of vehicles were seen departing around four hours later. The Yantian District People's Court said sentencing will be at a later date.

Court officials stayed tight-lipped about the case, and the trial -- like many in China's opaque legal system -- was not open to foreign reporters or diplomats.

At least two of the group could face around seven years in jail for organising the attempted escape from a city where democracy activists last year led massive protests against Beijing's rule.

Some of the group were already facing prosecution in Hong Kong, where a draconian national security law imposed by China has largely stamped out the city's protest movement.

I would like to urge the Yantian court to hand down the verdict as soon as possible... With a verdict I can set off for Shenzhen. I just want to meet my son, activist Wong Wai-Yin's mother said Monday evening.

Activist Lester Shum added that none of the families we know got to hear the trial, despite the court's statement saying relatives were present.

Their so-called crime was to flee tyranny, a US embassy spokesperson told AFP, ahead of the detainees scheduled court appearance.

Urging their immediate release, the spokesperson said: Communist China will stop at nothing to prevent its people from seeking freedom elsewhere.

The US has used Hong Kong's evaporating freedoms as a stick to beat China with in recent months.

China's Ministry of Foreign Affairs went into a familiar attack mode over the US comments.

Washington must immediately stop interfering in China's internal affairs through the Hong Kong issue and immediately stop interfering in China's judicial sovereignty, spokesman Zhao Lijian told reporters.

- Secret trial -

Families of the accused -- the youngest of whom is just 16 -- have called for the hearing in the Shenzhen court to be broadcast live, after they were unable to attend due to the short notice for the trial and Covid-19 requirements.