Unhappy at being told to stop smoking at a bus interchange, a man pushed an elderly part-time security officer to the ground, and then punched and kicked him as his colleague lay on top of the victim.

Two teachers who intervened were also assaulted.

Yesterday, Kuhaannath Pillai, 25, then a supervisor with a cleaning service company, was jailed for two weeks for causing hurt to Mr Abdul Hamid Abu Bakar, 73, and hitting teacher See Wan Ting in the back of her head at Pasir Ris Bus Interchange on Jan 23.

The case against his colleague, Dinesh Gopi Nathan, 28, then an operations executive, is pending.

Kuhaannath was also fined $1,000 for disorderly behaviour in a separate incident at a police station more than a month earlier.

Deputy Public Prosecutor Lu Yiwei said Mr Abdul Hamid told Kuhaannath, Dinesh and Mr Melvin Raj George Francis to stop smoking at the bus interchange at about 12.30am on Jan 23.

Kuhaannath pushed Mr Abdul Hamid's shoulder, causing him to fall. Dinesh rushed over and allegedly lay on top of the victim, while Kuhaannath assaulted him.

Bystanders Lim Chih Chang, 42, and Ms See, 23, shouted at them to stop. Dinesh and Mr Melvin Raj then helped the victim to a nearby bench.

But Dinesh then allegedly punched Mr Lim, a teacher, in his stomach. Upon seeing this, Ms See struck him with a bottle.

Kuhaannath then hit her in the head, causing her to fall. He also pushed Mr Lim and punched him.

Mr Lim and Ms See later went to a polyclinic with superficial injuries.

DPP Lu said Kuhaannath and Dinesh were drinking before the incident, and were on their way home.

Kuhaannath could have been jailed for up to two years and/or fined up to $5,000 on each charge of causing hurt; and for disorderly behaviour, fined up to $2,000 and/or jailed for up to six months.