Protestors took to the streets across Bangladesh Tuesday to demand the death penalty for the killers of a 13-year-old boy as anger mounted over the sickening lynching which was captured on video and then went viral.

Bangladeshi police have now arrested five people over the July 8 killing of Samiul Alam Rajon, who was tied to a pole and then subjected to a brutal assault in which he pleaded for his life.

One of those arrested was detained in Saudi Arabia after outraged members of the large Bangladeshi expat community there shopped him to police.

The 28-minute video of Samiul, which was widely circulated after being posted on social media, has prompted deep soul-searching among Bangladeshis as well as a series of mass protests.

Hundreds of people demonstrated at the scene of the murder in Samiul's home city of Sylhet on Tuesday morning demanding that the killers be sent to the gallows.

"Protesters held hands at the site of the murder. They were demanding execution for the killers," local police chief Akhter Hossain told AFP.

A new mass demonstration has been called for Tuesday afternoon in the centre of the city in the country's northeast.

Protests were also held in the capital Dhaka where hundreds shouted "Hang them, hang them" as they massed outside the national press club, an AFP photographer at the scene said.

Although executions are rare in Bangladesh, several Islamist leaders have been hanged in recent months over their role in the 1971 independence war.

Thousands of demonstrators held similar protests on Monday, venting their anger at police for not doing enough to hunt down the killers.

Samiul was accused by his attackers of stealing a bicycle although his family insist that the youngster was innocent.

Police said officers had arrested three people overnight, including the wife of one of the suspects and two men who were at the scene. A court remanded one of them to five days in custody.

Police were also hunting for "two others who directly took part" in the beating, police chief Hossain told AFP.

"It's a brutal and heinous murder and we won't spare anyone," he added.

SAUDI ARREST

The latest arrests bring the number of people in custody to five, including the alleged ringleader Muhit Alam and his main accomplice Kamrul Islam who was arrested in Saudi Arabia, according to Bangladesh's foreign ministry.

Islam fled to Jeddah in the immediate aftermath of the attack but reports said he was arrested by the Saudi authorities after members of the large Bangladeshi expat community in Jeddah reported him to police.

In a video filmed by an expat TV station as he was led away to a waiting police car in handcuffs, Islam said that he had "made a mistake, please forgive me," before bursting into tears.

An autopsy on Samiul found 64 separate injuries had been inflicted on the teenager during the assault in which he could be seen begging for water.

In the video, the terrified youngster can be heard screaming in pain and repeating: "Please don't beat me like this, I will die."

Samiul's attackers can be heard on the footage trying to force him to confess his involvement in the theft.

At one stage he is told to walk away. But as he tries to get to his feet, one of the attackers shouts: "His bones are okay. Beat him some more."

It is not clear who filmed the video although police initially said they believed it was captured on smart phone by one of the attackers.

Police said dozens of people witnessed the murder but none tried to stop the killers or report the incident to police.

"It's unfortunate that none of them came to us. Some 20-25 people witnessed the horrible crime and and they did nothing," deputy commissioner of Sylhet police Faisal Mahmud told AFP.