The trial in China of a group of Hong Kong pro-democracy activists who tried to escape the city by speedboat for sanctuary in Taiwan opened 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 facing 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, AFP understands, and was likely to run into the evening.

But court officials would confirm nothing 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 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 China has imposed a draconian national security law that has stamped out the city's protest movement.

"Their so-called 'crime' was to flee tyranny," a US embassy spokesperson told AFP, ahead of their scheduled court appearance via video link.

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.

They were only notified of the trial date on Friday, while their lawyers have been barred from meeting with the detainees. Authorities instead have appointed state-approved legal representation.

In a joint letter over the weekend, the families said they "strongly condemn" the authorities' decision to hold the trial in "de facto secret" at Yantian District People's Court.

"We urge governments to send embassy personnel to the hearing to guarantee a proper and fair trial by the courts in Shenzhen," they said, noting that those detained include British, Portuguese and Vietnamese nationals.

The security law that now blankets Hong Kong gives authorities sweeping powers of prosecution for acts deemed terrorism, secession, subversion or collusion with foreign entities.

The city had enjoyed unique freedoms since its handover from former colonial power Britain in 1997, with a deal promising a "one country, two systems" arrangement for 50 years.

Beijing says the new security law was needed to restore peace and stability but critics have condemned it as a fatal attack on Hong Kong's freedoms.

China has a history of putting dissidents on trial around the Christmas and New Year period to avoid Western scrutiny.

Amnesty International said there was little chance the group would receive a fair trial.

"They have so far been deprived of their basic rights, including the right to defend themselves through legal representation of their own choosing," Amnesty Hong Kong's Programme Manager Lam Cho Ming said in a statement.

Eight of the group are accused of an illegal border crossing, while two are suspected of organising for others to cross the border.

Two minors face non-public hearings.

Since Beijing imposed the security law in June, Taiwan has emerged as a sanctuary for Hong Kong activists -- quietly turning a blind eye to dissidents turning up without proper visas or paperwork.