The Golden Tours bus had ploughed into a souvenir stand as it turned the corner near the Admiralty pub at around 12.30pm.
Images on social media showed key rings, tote bags, and T-shirts scattered around the scene of the crash,
The same stand was then hit by a black 4x4.
A witness told the Sun: “I passed on the way home from work just as they were putting up the tape
“By the time I saw it, nobody was on the bus. But it was quite a way off the road, it was unexpected.
“The stall had been crippled.
“My initial thought was to wonder how I’d get through to get home from work, it was only when I got closer that I realised it was worse than I thought.”
The London Fire Brigade was called to the scene of the collision.
A spokesperson for Golden Tours said an investigation was underway to determine the circumstances of the crash.
In a statement, the tour operator said: "The wellbeing and safety of our passengers, staff and members of the public are always our highest priority
"Incidents of this nature are extremely rare, and we take all matters relating to safety very seriously.
"We will be in a better position to provide further information once we have established the full facts."
In a statement, the London Fire Brigade said: “Firefighters are in attendance at a road traffic collision on Spring Gardens in Charing Cross.
“One tourist bus was in collision with street furniture.
“The Brigade was called at 1232. Two fire engines and a Fire Rescue Unit from Euston, Paddington and Chelsea fire stations are attending the scene.”
The crash caused significant disruption across Trafalgar Square and the surrounding areas on Thursday afternoon.
The bus was removed from the scene of the incident around 5 hours after the crash, with recovery operations complete by 5.43pm.
Spring Gardens remained closed for infrastructure repairs, while traffic around Trafalgar Square was partially restricted.
By 8.11pm, congestion was reported on all approaches to Trafalgar Square, including delays stretching along Horse Guards Road towards The Mall.
At 8.26pm, westbound restrictions remained in place as repair teams continued working on damaged infrastructure.
Drivers were warned of further delays into the evening.