A smoke break turned into a trip to the hospital for sales executive Zhong Wen Cai on Tuesday after National Environment Agency (NEA) officers caught him littering.
Mr Zhong, who is in his late 20s, had gone to the smoking area outside Raffles City Shopping Centre to have a puff at about 11am.
He sat on a stone seat and when he got up to leave, he left the cigarette butt on the seat.
Two NEA officers who spotted his action approached him and told him that he was littering.
They asked for his identification card (IC), but he refused to comply.
Instead, he said, he picked up the cigarette butt, threw it into a rubbish bin nearby and tried to tell to the officers that it had been "a misunderstanding".
Claimed Mr Zhong: "I didn't throw the cigarette butt on the floor. I merely left it on the seat.
"Moreover, when they approached me, I had just stood up. I hadn't even taken a step forward."
Grabbed his shoulder
But the officers wouldn't let him off, he said.
And when he tried to walk away, one of them grabbed his right shoulder, he said.
In a letter that he sent to NEA, Mr Zhong claimed the officer used so much force that it hurt, and he swung his right arm to free himself.
He said that he then tried to walk faster to get away, but an officer ran up and knocked him to the ground.
He claimed the officer pinned him down with a knee on his back and locked his left arm behind his back.
He said that even when he relented and agreed to surrender his IC, he was held down for another two minutes before he was finally released.
He then handed over his IC and driving licence to the officers.
He described the officers' treatment of him as an "assault" and said he was publicly embarrassed.
In his letter to NEA, a copy of which was sent to TheNew Paper, Mr Zhong said he "felt like a criminal" and did not understand why the officers had to "resort to such violence".
He called the police and was taken by ambulance to the Singapore General Hospital, where he was treated for bruises on his shoulder and hand.
He claimed he was in intense pain from the injuries on his left arm and he was given three days of medical leave.
When contacted, an NEA spokesman said that the case has been referred to the police, who have classified it as one where criminal force was used against a public servant.
Investigations are ongoing.