SINGAPORE - Using spray paint, they vandalised cars, walls and letter boxes.
In bright colours, they sprayed "O$P$", which stood for "Owe money, pay money", and peppered walls with profanities.
The four vandals are primary school pupils and when they went on their vandalism spree on Tuesday near Block 168C, Punggol Field Road, they were in their school uniforms.
Mr Benjamin Lei, who was visiting his mother-in-law living in the vicinity, spotted four boys behaving suspiciously.
Mr Lei, a 33-year-old sales executive, recounted: "When they saw me, they kept saying 'Quick, quick! Someone is here.'"
The boys walked past him briskly and avoided eye contact with him.
One of them said: "Hide the bottle."
Mr Lei heard a "clack clack clack" sound, which he later realised was the sound made by the ball bearing inside a spray can.
When they realised Mr Lei was on to them, the boys sprinted, but he caught up with them after about a 100m chase.
By then, he realised that the boys had vandalised several cars.
He called the police while holding on to the boys, who were aged between nine and 10.
Mr Lei, who is the father of a year-old boy, questioned them while waiting for police officers to arrive.
"They said they didn't do anything," recounted Mr Lei who had parked his car at the same multi-storey carpark.
"They brushed me off when I asked them what was in their bags," he added.
One of the four boys even tried to escape by telling Mr Lei that he needed to visit the washroom.