LONDON, United Kingdom - Swedish campaigner Greta Thunberg on Wednesday denied a public order offence, after she was arrested at a demonstration against the energy industry in central London.

The 20-year-old activist -- a key face of the movement to fight climate change -- was among 26 people charged at the October 17 demonstration in the British capital.

Inside Westminster Magistrates Court, Thunberg, who was mobbed by photographers and camera crews as she arrived, giggled with other defendants as she waited for the hearing to start.

She spoke only to confirm her name, age and correspondence address, and to enter a plea of not guilty.

Four other defendants appearing alongside her also pleaded not guilty.

They were given unconditional 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 Greenpeace and Fossil Free London campaigners earlier stood outside the court with banners proclaiming "Oily money out" and "Make polluters pay".

A 28-year-old charity worker from Fossil Free London who gave her name only as Josie told AFP she was there to show solidarity with those arrested.

Last month's meeting involved oil executives and others for discussions about making "lots of money out of destroying our future", she added.

The demonstration saw several hundred rally outside a major London hotel, blocking all entrances to the venue.

Message

Maja Darlington, a campaigner at Greenpeace UK, said the group's activists joined the demonstration to send "a clear and peaceful message" to the oil bosses.

Before her arrest, Thunberg had criticised "closed door" agreements 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.

Thunberg was put in a police van outside the Energy Intelligence Forum.

She has since come under fire for wearing a black and white Palestinian scarf and urging a "ceasefire now" during a climate protest in Amsterdam.

Her speech at the event on Sunday was interrupted by a man who tried to snatch a microphone from her, saying he had come for a climate protest, not to hear her other views.

After he was removed by security officers, Thunberg began chanting, along with the crowd, "no climate justice on occupied land".

Thunberg, who started the so-called "School Strike for Climate" movement as a teenager, was fined by a court in Sweden in October.

That was imposed after she was convicted of having resisted arrest during a July protest that blocked traffic.

In court on Wednesday another another five people also charged over the protest appeared after Thunberg and denied the same offence.

They will face trial at a later date.