A fire broke out at Shenton House in the Central Business District around lunchtime yesterday.
Two women - one in her 20s and the other in her 50s - were taken to Singapore General Hospital after they inhaled smoke.
The fire, which started in a unit on the 15th floor of 3 Shenton Way, was put out with a water jet.
Mr Muljadi Tantra, 46, who works in a trading company in the building, smelt smoke in his office on the 20th floor at around 1pm.
"I thought it was coming from my own office but I checked the electric plugs and couldn't find a source. Later, my colleagues and I started seeing smoke coming out of the vent so we decided to evacuate," he added.
They alerted people working in other offices on the same floor and made their way down the building, using the stairs.
Mr Nick Wang, 32, a boarding officer who works on the 24th storey, said the building management went to each floor to alert occupants to the fire. He added: "The intercom also had an announcement saying: 'This is not a drill. Please evacuate the building.'"
As he was passing the 15th floor on his way down the stairs, Mr Wang saw thick smoke in the corridors.
Around 150 people were evacuated from the 25-storey building.
The Singapore Civil Defence Force (SCDF), which was alerted to the fire at about 1pm, sent three fire engines, a Red Rhino, two ambulances and two support vehicles to the scene.
When its officers arrived, they found that the affected floor was full of smoke.
The officers broke into the affected unit to extinguish the fire.
The cause of the blaze is being investigated.