A Thai court has convicted nine young Muslim men from southern Thailand of planning to set off a car bomb in the capital, in a case in which many of the defendants said they were tortured into making false confessions.

The Criminal Court in Bangkok on Tuesday found all nine guilty of belonging to an underground criminal group and sentenced each to four years' imprisonment.

One was also found guilty of illegal possession of explosive devices, giving him a total of six years in prison.

Five defendants were acquitted.

The case began when at least 50 Thai Muslims, mostly students at Bangkok's Ramkhamhaeng University, were rounded up in joint police-military sweeps on Oct 10, 2016.

They were released but 13 were rearrested the following month along with an additional suspect.