weapons fashioned from smashed up beds and broom handles after seeing pictures of religious violence in Myanmar, in March, that left dozens dead.
The attack underscores the soaring Muslim-Buddhist tensions that have cast a shadow over political reforms in Myanmar, where the end of decades of authoritarian military rule has laid bare deep sectarian fault lines.
March's disorder was the worst since an eruption of violence between Buddhists and Rohingya Muslims in the western state of Rakhine in 2012 that left scores dead and tens of thousands - mainly Muslims - displaced. An increasing number of Rohingya - viewed by many Burmese as illegal Bangladeshi immigrants - have been arriving on Indonesian shores.
Immigration centre official Rida Agustian said the entrance to the block where the detainees were being held was sealed off to stop officers from intervening as the deadly attack was launched in the early hours of Friday. When officers finally got in, they found "blood spattered on the walls and in pools on the floor," he said.
"The men had used wood from their beds and broom sticks as weapons to kill. The bodies were covered in blood, it looked like they were beaten and tortured to death."
Agustian said 15 people, believed to be Rohingya, were injured during the violence at the centre, where 106 of them are being held. Kyawkyaw, 25, who gave only one name and is one of a handful of Myanmar Buddhists being held at the centre, said he heard the attack unfolding and was "very scared". "We ask the Indonesian government to send us straight back home to Myanmar," he told AFP outside the centre before being taken away for questioning by police.
He said the victims were from a group of Myanmar Buddhists being held at the centre at the port of Belawan after being caught illegally fishing in Indonesian waters, and had been due to be deported in May.
The attack began after the detainees saw images of recent violence between non-Rohingya Muslims and Buddhists in central Myanmar that has left at least 43 people dead and many Muslim homes and mosques destroyed, said local police chief Endro Kiswanto. "They managed to see some photos of the violence in Myanmar, including buildings on fire, and we believe that's when the violence broke out," Kiswanto told AFP.
He said all eight Buddhist men were dead when police arrived at the detention centre in the early hours of Friday morning. 25 detainees and 30 other witnesses were being questioned by police, and 30 officers were at the site, Kiswanto said.
Many Rohingya Muslims arriving on Indonesian shores face long stints in detention awaiting UN assessment for refugee status. Myanmar views its population of roughly 800,000 Rohingya as illegal Bangladeshi immigrants and denies them citizenship.
The communal violence in March was apparently triggered by an argument in a gold shop in the central town of Meiktila that turned into a riot, but witnesses say the wave of violence since then appears to have been well organised.
Human Rights Watch (HRW) this week urged Myanmar to investigate the failure of police to stop the violence. "The government should investigate responsibility for the violence in Meiktila and the failure of the police to stop wanton killings and the burning of entire neighborhoods," said HRW Asia director Brad Adams.