London, England (CNN) -- Greenpeace protesters briefly shut down BP stations across London on Tuesday, after the oil company announced its second-quarter results, BP and Greenpeace said.

Greenpeace said it had shut down 46 stations, but BP said the number was 40.

The protesters dropped off a letter at each station in the morning and, on their way out, pulled a safety switch that cut off power to the station, BP and Greenpeace said.

They also covered BP signs with posters reading, "Closed: Moving beyond petroleum," a take on BP's slogan.

Most of the stations were operating again by the afternoon and the rest were expected to reopen by the evening, BP said.

One of the stations supplied ambulances, BP said.

The affected stations were a mix of company-owned and dealer-owned, BP said. Greenpeace said it simply targeted stations that sell BP gas.

Asked about targeting stations that may be owned by individuals or businesses other than BP, Greenpeace spokeswoman Beth Herzfeld said they "don't want to make anybody suffer."

"We're sorry for any problems that are caused, but the petrol that are sold in these businesses are BP," she told CNN. "BP is determined to chase the last drops of oil, whatever the cost to the natural environment and people's health."

Herzfeld said Greenpeace was aware that one of the targeted stations supplies ambulances, but she said the group was directing emergency vehicles to other non-BP stations nearby.

"We're very careful and very keen to make sure that nobody's safety is compromised," she said.

The protests came after BP unveiled a massive $17.15 billion loss in the second quarter due to costs stemming from the Gulf of Mexico oil disaster.

The oil company said it took a pre-tax charge of $32.2 billion in the quarter to cover costs relating to the oil spill.

It also announced its embattled Chief Executive Officer Tony Hayward will step down on October 1, following fierce criticism of the way he has handled the disaster.

CNN's Kendra Petersen and Caroline Paterson contributed to this report.