The fire broke out in the pub The Admiralty shortly before 5.50 pm on Tuesday and huge plumes of smoke came from the building.
Initially about 10 fire trucks and 70 firefighters were called in, but this was later expanded under “severe” conditions to 20 fire trucks and about 125 firefighters.
The London Fire Brigade said half of the pub’s basement was on fire but had the fire under control by 10pm.
firefighters out Soho and surrounding fire stations attended the incident.
Station Commander Keith McDermott, who is on site, said: “Due to the severe, challenging and hot conditions in the basement, we have increased the number of fire engines and firefighters on the scene.
“About 150 people evacuated the pub and surrounding businesses before the brigade arrived.”
There are no reports of injuries and the cause of the fire is under investigation.
The fire comes amid a heat wave, with temperatures soaring to 30°C in the capital on Tuesday.
The fire brigade has given an update on Twitter about three hours after firefighters started fighting the flames.
A spokesman said: “Constant progress is being made with the fire in the pub in Trafalgar Square. We will probably be on site for hours.
It’s a hot night, so the crews are taking in a lot of water. Please think of all firefighters dealing with fires in these circumstances.”
The pub said on Twitter: “Due to the current situation, The Admiralty is closed until further notice. We will update our website and social channels in due course.”