Dozens of people arrested in Thailand's human trafficking crackdown, including a senior general, appeared in court Tuesday, days after the police officer in charge of the investigation tendered his resignation saying he feared for his life.

The kingdom has long been a major hub for human trafficking and people smuggling, with rights groups accusing officials of turning a blind eye to the grim trade -- and even complicity in it.

Thailand's junta launched a belated crackdown in May, a move that uncovered death camps on the Thailand-Malaysia border and led to dozens of arrests but also saw thousands of migrants abandoned on land and at sea by gang masters, eventually forcing a region-wide response.

Among those who appeared in a Bangkok court Tuesday for the procedural hearing was senior army general Manas Kongpan who is charged with being a linchpin of the trade.

At least four other military officers have been implicated, two of whom have yet to be arrested. Local politicians and business figures are also among those charged.

Manas's lawyer said his client was innocent.

"We deny all the charges and I have prepared a dozen witnesses in this case," attorney Noppachai Veratanya told reporters at the court.

The crackdown came after Thailand was embarrassingly downgraded by Washington in 2014 to the lowest category of an annual State Department report that evaluates countries' willingness to tackle human trafficking.

Thailand's junta has trumpeted the recent arrests as proof that the kingdom will no longer tolerate the trade and will pursue complicit officials no matter how powerful they are.

However, the junta's rhetoric has been thrown into doubt by the senior police officer who led the trafficking investigation.

In an interview with AFP last month, Major General Paween Pongsirin said his investigation was closed too quickly, with many suspects still at liberty and his unit disbanded.

He also said he feared for his life after implicating senior military figures in the grisly, multi-million dollar trade.

Over the weekend he handed in his resignation after superiors ordered him to take up a new post in Thailand's deep south, an insurgency-plagued region where the army effectively controls security.

Thailand has long been a hub for the trafficking of persecuted Rohingya Muslims from Myanmar, who in recent years have been joined by Bangladeshi economic migrants.

With the monsoon ending, rights groups warn that boats are likely to set sail in the coming weeks.

Thailand has remained on the bottom tier of the State Department's trafficking list for a second year in a row, alongside nations like Iran, Libya, North Korea and Syria.