A Chinese court has sentenced agricultural tycoon Sun Dawu to 18 years in jail for a catalogue of crimes including "provoking trouble" after the outspoken billionaire and grassroots rights supporter was tried in secret.

The court in Gaobeidian near Beijing said Sun was found guilty of crimes including "gathering a crowd to attack state organs," "obstructing government administration" and "picking quarrels and provoking trouble," a catch-all term often used against dissidents.

He was detained by police in November along with 19 relatives and business associates after his firm was embroiled in a land dispute with a state-owned competitor.

The charismatic Sun built one of China's biggest private agriculture companies with his wife from a few chickens and pigs in the 1980s.

He has also been a vocal champion of rural reforms and a whistleblower during a devastating swine fever outbreak in 2019, posting photos of dead pigs online after local officials were slow to respond to the disease.

Sun was also fined €400,000 by the court.

Sun Dawu built one of China's biggest private agriculture companies from a few chickens and pigs

The hearing began at Gaobeidian People's Court in northern Hebei province last Thursday, according to his lawyers, who said in a statement that the secrecy of the trial "violated legal guidelines and did not protect the defendant's litigation rights."

Sun had previously been sentenced to prison for "illegal fundraising" in 2003, but this was overturned after a massive outpouring of support from human rights defenders and the public.

Video footage shared by Sun's lawyer showed tight security outside the courtroom, with convoys of police vehicles and plainclothes police.

Prosecutors alleged the Dawu Group deceived its employees, "seriously interfered with local orderly administrative management" and "endangered national grassroots political stability", according to a court witness account shared by his legal team.

In response, Sun said the Dawu Group was "wholly socialist, everyone is on the road to common prosperity, and Dawu employees live very well," according to the account.

The Dawu Group employed over 9,000 people before Sun's assets were seized by the state and employees forced out of after Mr Sun's detention in November.

The businessman has for decades been a vocal critic of China's rural policies and has demanded greater freedom for farmers to organise to protect their economic interests.

He ran a website in the early 2000s that included criticism of state-owned banks, which he accused of neglecting rural investment while funnelling rural residents' savings toward urban projects.

His farms were badly affected by the 2019 African swine fever outbreak, which decimated pig herds nationwide.

He criticised the Hebei local government for attempting to cover up the scale of the outbreak and posted pictures of thousands of dead pigs online, quickly forcing it to respond.