Activists occupied the British Gas headquarters in Surrey for more than seven hours to protest at fuel bills.
Six people "barricaded themselves into meeting rooms" at about 09:00 GMT, Fuel Poverty Action said.
The protesters said they left the building on The Causeway trading estate in Staines at about 16:30 GMT because of a "threatening police presence".
Both British Gas and Surrey Police said they had worked together to ensure the safety of workers and the public.
On Twitter, a member of the group wrote: "Decided to leave on our own terms following threatening police presence. Let's hope this is only the beginning of challenging the big six!"
But a spokeswoman for Surrey Police said: "We were in attendance to ensure the safety of the public and the workers and the protesters themselves.
"We asked them to leave and they left of their own accord."
The group said in a statement they had targeted British Gas as it was one of the "big six energy firms making profits out of rising energy bills".
Hannah Edler, a 27-year-old protester, said: "We could have a fairer system where our energy is owned by communities who decide how it is priced and produced."
A British Gas spokesman said the firm recognised the impact of higher energy prices on household budgets and had introduced a series of measures to help customers through the winter.
He said it had spent more than £500m on helping vulnerable customers over the past three years.