He was manning his security post at a construction site at Jalan Tukang in Jurong when he saw workers running to the other side of the site.
Curious, Mr Anwar Husson followed them to an area near the crane where a crowd of workers had already gathered.
As the security guard peered over the heads of the people in front of him, he saw a shocking sight: There was a man lying in a pool of blood on the ground.
Mr Anwar, 25, said: "Blood was flowing from the back of his head, his eyes, nose and mouth. It was really scary."
Near the man was a wooden plank about two metres long, he said.
Mr Anwar recognised the victim when he took a closer look - an acquaintance who was a construction worker at the site.
"He used to hang out with my friends and me at work, and we would talk for a bit. He was really friendly even though he was much older than us," he said.
Workers at the construction site identified the victim as Mr Ali Rahmat, 39, a Bangladeshi national.
In the incident that happened at about 6.40pm on Monday, it is believed that he was struck by the wooden plank, which weighed about 15kg and had dropped from the sixth storey of the building.
According to Chinese evening daily Lianhe Wanbao, Mr Ali had been working in Singapore for eight months at the construction site for an industrial building to be completed this year.