They sang and held up banners as Ms Thunberg, a global face of the climate change movement, pleaded not guilty.
The 20-year-old activist was among 26 people charged with public order offences after police imposed limits on the demonstration in London, near the InterContinental Hotel in Mayfair, where oil executives were meeting for the Energy Intelligence Forum.
She pleaded not guilty at Westminster Magistrates' Court to blocking the hotel entrance during the protest.
She spoke only to confirm her name, age and address and to enter the not guilty plea. Four other defendants appearing alongside her also denied the charges against them.
They were given bail until an expected two-day trial that the court was told would start at City of London Magistrates' Court on February 1.
Climate protesters from Greenpeace and Fossil Free London stood outside the court with banners proclaiming "Oily money out" and "Make polluters pay".
Ms Thunberg was surrounded by photographers and film crews as she arrived at court.
A 28-year-old charity worker from Fossil Free London said she was there to show solidarity with those arrested.
Greenpeace campaigner Maja Darlington said the group's activists joined the demonstration to send "a clear and peaceful message" to the oil bosses.
Before her arrest, Ms Thunberg had criticised "closed door" agreements struck between politicians and representatives of the oil and gas industry.
London police said they imposed "conditions to prevent disruption to the public" at the protest, which were then breached, prompting the arrests.
Ms Thunberg was among those put in a police van outside the Energy Intelligence Forum.
Several hundred people rallied outside the luxury hotel, blocking all entrances to the venue.
Ms Thunberg, who started the School Strike for Climate movement as a teenager, was fined by a court in Sweden in October, after being convicted of resisting arrest during a July protest that blocked traffic.