The six were detained as they tried to hang a banner reading "Listen to the People, not the Polluters" at a Durban hotel where a "Global Business Day", hosted by business organisations, was taking place.
The protest aimed at a so-called "Dirty Dozen" corporations which Greenpeace says are pulling political strings to stifle progress toward a global climate deal.
The incident happened outside Durban's International Conference Centre (ICC), which has been declared UN territory for the duration of the 12-day marathon.
Police confirmed that the activists have been arrested, but could not confirm what was written on the flag.
"I can confirm that seven Greenpeace environmental activists have been arrested for trespassing," Colonel Vish Naidoo said.
They allegedly entered the Marine Parade hotel on Durban's beachfront and attempted to put up a flag, he said.
"I don't know what was written on the flag."
The talks in Durban are taking place under the flag of the 194-nation UN Framework Convention on Climate Change (UNFCCC).
The forum has drawn 15 000 people, including government delegates, campaigners, business and the media.
The meeting, ending on Friday, has been struggling to make headway on apportioning curbs in greenhouse-gas emissions, blamed for dangerous climate change.
Greenpeace activists are attending COP 17 talks in the city. Greenpeace was not immediately available to comment.