Simply put, power lies in the Party. The control the physical power (the can start/arrange for convenient wars that increase dependency on the government, and perhaps more importantly, they control "informational" power. You can never know anything for sure in their state. They control the present and past, so they can control the future by inventing an appropriate past.
This is all clear in the "reeducaton" scene with O'Brien at the end of the novel. He explains to Winston the the party holds power for a different reason that parties did in the pst. They always claimed that they held power to do good for the people, blah, blah, blah. O'Brien makes it clear that in 1984, the Pary hold power for power's sake. Hitler, Stalin and older dictators failed because their either did not understand this, or they did not act according to their knowledge if they had it. O'Brien suggests that since reality exists in the mind of each perceiver, and that since the Party controls the mind of each individual, that the Party controls reality ... that's REAL power.
You can find additional examples at the enotes source listed below. You might also want to reread the chapter to look for additional examples ....