Police said the incident arose after a man was sprayed with an unknown substance by two assailants.

The Singapore Civil Defence Force (SCDF) said it received a call just after 4pm informing them that someone had collapsed and there was a smell of gas in the North Bridge Road building.

About 40 SCDF officers, seven ambulances, a Hazmat or 'hazardous materials' response team, two fire engines and a Red Rhino were sent to the scene. By the time they arrived, the building's security officers had evacuated about 100 tenants and shoppers.

The SCDF ran tests to determine what the mystery gas was, but was unable to detect any trace of it, a spokesman said.

The all-clear was given at 5.45pm and the building was re-opened.

The 18 people - six men, 11 women and an infant - were taken to Tan Tock Seng Hospital and Singapore General Hospital. Most were kept for observation overnight.

Several tenants told The Straits Times that they had heard an argument near the entrance of the building at around 4pm. Shortly after, some began having difficulty breathing and rushed outside, where they saw a man lying on the ground, foaming at the mouth.

Said Mr Nizam Deem, 50, who owns a textile shop on the ground floor: 'I came out of my shop when it became difficult for me to breathe.'

A spokesman for the building's management who had viewed closed-circuit camera footage confirmed that a dispute had taken place between two middle-aged men, and said the gas incident appeared linked to it.

He refused to elaborate, but said: 'The two men are not tenants of the building and we have never seen them before.'

Police are investigating.