Climate activist Greta Thunberg arrested at London protest outside ...

Greta Thunberg has been detained by UK police alongside other climate activists who gathered outside a central London hotel to disrupt a major oil and gas industry conference.

Key points:Protesters attempted to block access to the conference venue by sitting by the entranceTwenty-nine people were arrested during the protestsThe protesters accuse fossil fuel companies of deliberately slowing the transition to renewable energy

Ms Thunberg was among dozens of protesters who on Tuesday chanted "oily money out" and sought to block access to the luxury InterContinental Hotel on Park Lane, which is hosting the Energy Intelligence Forum.

The conference features speakers including the chief executives of Shell, Saudi Arabia's Aramco and Norway's Equinor, as well as the UK's energy security minister.

An Associated Press photographer saw officers speaking with Ms Thunberg before leading her away and taking her into a police vehicle.

Protesters attempted to block access to the conference venue by sitting on the pavement by the entrance.

They held aloft banners and chanted "oily money out" and "cancel the conference", while some lit yellow and pink smoke flares.

Two Greenpeace activists abseiled down from the roof of the hotel to unfurl a giant banner reading "Make Big Oil Pay".

Greta Thunberg accused the fossil fuel companies of "greenwashing and lies". (AP: Kin Cheung)

London's Metropolitan Police said 29 people were arrested at Tuesday's protests, including six on suspicion of obstructing a highway and 21 others for breaching protest conditions.

One person was detained on suspicion of criminal damage. All were in custody.

Police said they engaged in conversations with the protesters about allowing people to access the venue safely and prevent serious disruption to the hotel and guests, but some of the activists refused to move from the road.

No charges have been issued yet.

"We were linking arms when the police forced their way in and singled out Greta. She was dragged down the street at speed to a police van where they refused to say where she was being taken," said Joanna Warrington, an organiser with Fossil Free London who was at the demonstration.

The protesters accuse fossil fuel companies of deliberately slowing the global energy transition to renewables in order to make more profit.

They also oppose the UK government's recent approval of drilling for oil in the North Sea, off the Scottish coast.

UK authorities have defended the move, saying it is necessary for the country's energy security.

Greta Thunberg said she believed oil and gas companies had no intention of transitioning to renewables. (AP: Kin Cheung)

"The world is drowning in fossil fuels. Our hopes and dreams and lives are being washed away by a flood of greenwashing and lies," Ms Thunberg told reporters before she was detained.

"It has been clear for decades that the fossil fuel industries were well aware of the consequences of their business models, and yet they have done nothing."

"We cannot let this continue. The elite of the oil and money conference, they have no intention of transition," she added.

"We have no other option but to put our bodies outside this conference and to physically disrupt. And we have to do that every time, we have to continue showing them that they are not going to get away with this."

Police said those protesters detained were taken into custody and that officers remained on site.

Environmental activists plan to protest outside the Intercontinental Hotel throughout the three-day conference. (AP: Kin Cheung)

Environmental groups say they will continue to protest throughout the planned forum, which is expected to last three days.

Ms Thunberg inspired a global youth movement demanding stronger efforts to fight climate change after staging weekly protests outside the Swedish parliament starting in 2018.

She was recently fined by a Swedish court for disobeying police during an environmental protest in Sweden.

AP