Daniel 8:23-25
"And in the final part of their kingdom, as the transgressors act to a completion, a fierce-looking king who understands ambiguous sayings will stand up."
The UN leader is the king referred to here.
"His power will become great, but not through his own power. He will bring ruin in an extraordinary way, and he will be successful and act effectively. He will bring mighty ones to ruin, also the people made up of the holy ones."
The UN will attack religious institutions, which will also affect faithful individuals.
"And by his cunning he will use deception to succeed; and in his heart he will exalt himself; and during a time of security he will bring many to ruin. He will even stand up against the Prince of princes, but he will be broken without human hand."
At the end of the great tribulation, Jesus will come and put an end to the rulership of the UN leader, and all the other world leaders.
The progression of world powers is described in parallel accounts at Daniel chapter 2, Daniel chapter 7, Daniel chapter 8, Daniel chapters 11 and 12, and in Revelation.
When taken all together, it is clear that the UN will be the dominant world power for 3 1/2 years during the time Jesus has received the kingdom in heaven.