BANGKOK -- Thailand's police chief announced the arrest of a powerful mayor on Friday and said 50 police officers are under investigation in a widening human trafficking scandal spanning four Asian countries.

At an urgently called meeting of senior police from around Thailand, police chief Gen. Somyot Poompanmoung delivered the strongest public admission yet of police involvement in trafficking syndicates that use Thailand as a regional transit hub. Human rights groups have long accused Thai authorities of collusion in the trafficking industry but the claims were routinely denied by police.

"If you are still neglecting, or involved with, or supporting or benefiting from human-trafficking networks -- your heads will roll," Somyot told the meeting at Bangkok's national police headquarters.

Last Friday, police unearthed two dozen bodies from shallow graves in the mountains of southern Thailand, a grim discovery that has since exposed a network of jungle camps run by traffickers who allegedly held migrants captive while they extorted ransoms from their families. A total of 33 bodies, believed to be migrants from Myanmar and Bangladesh, have now been exhumed from various jungle camps.

The discoveries have embarrassed Thailand, which is already under pressure from the United States and the European Union to crack down on human trafficking both on land and in its fishing fleets.

Authorities say they have known for years that the area on the Thai-Malaysia border was used to smuggle Rohingya Muslims, a persecuted minority in neighbouring Myanmar, as well as Bangladeshis and other migrants, to third countries including Malaysia, which is predominantly Muslim.

Rohingya Muslims have taken to the sea in greater numbers in recent years to escape violence and state-sanctioned discrimination in Myanmar. Mob attacks in the last three years have left hundreds dead and sparked one of the region's biggest exoduses since the Vietnam War, which has fueled a thriving industry for traffickers.

The head of Thailand's military-controlled government, Prime Minister Prayuth Chan-ocha, called Friday for a meeting with neighbours Malaysia and Myanmar, saying Thailand cannot solve the problem alone. He reiterated a pledge that any officials linked to the criminal networks will be prosecuted.

"We have to punish the human traffickers strictly, according to the law," Prayuth said. "If any government officials or authorities are involved, they will face punishment."

In a country known for pervasive corruption, many in Thailand have reacted with horror but not surprise to the apparent failure of authorities to stop the trafficking. International human rights groups have called for an independent investigation to uncover the truth, given the alleged involvement of authorities.

In an editorial Friday titled "Take Down the Traffickers," the Bangkok Post said the arrests confirm "long-held public suspicions of government officials' collusion with trafficking networks."

A swift crackdown has included the arrests of eight people -- mostly local officials and police -- for suspected involvement in the syndicates. Among them was Friday's arrest of the prominent local mayor of Padang Besar, the sub-district of southern Songkla province where most of the bodies were exhumed.

Somyot called Mayor Banjong Pongphon a "key suspect" in the probe and said the politician, who has been in office over a decade, "wielded great influence" in the region. Banjong denied charges of committing human trafficking, providing assistance for illegal aliens entering the country, detention and ransom.

The day before his arrest, Banjong joined a march in Padang Besar that drew hundreds of people and government officials in calls to oppose human trafficking.

Additionally, more than 50 police have been transferred out of their posts pending an investigation, including officers in charge of anti-trafficking, immigration, marine surveillance and border units in the area of the camps.

"To remove about 50 officers from their posts isn't something I want to do, but it's something I have already warned you about," Somyot told the meeting of police officials. "I have warned but you didn't listen. I have warned but you still did it."

Somyot acknowledged criticism that in the past police who have been investigated for trafficking and other crimes have gone unpunished but vowed "this time it will be different," noting that the world was watching.

"Overseas countries, especially the United States, are monitoring us," Somyot told his officers.

Last June, the U.S. downgraded Thailand and Malaysia to its lowest category -- Tier 3 -- in an annual assessment of how governments handle human trafficking. Thailand promised action to get off the blacklist, but its reputation suffered more following recent revelations by the AP that some Thai fishing vessels kept men from Myanmar, Cambodia and Laos on board as forced labour or slaves.

Australian Foreign Minister Julie Bishop called the discovery of graves "shocking" and "a terrible tragedy" during an official visit Friday.

"I acknowledge the fact that the Thai government has responded in a timely fashion," Bishop told reporters. "That there is to be an investigation, we certainly welcome that."

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Associated Press writer Jocelyn Gecker contributed to this report.