Jesus makes use of a perfect participle that should be translated as they will witness the fact that the kingdom has come with power.
The focus is not on the arrival but the time afterwards when its reality is visible and displayed with power. Generally, this verse is understood in one of three different ways.
The kingdom coming with power could be a reference to Jesus’ death and resurrection.1 I think this is the most likely meaning given the fact that Jesus is speaking to the crowd and the disciples.
The kingdom coming with power could refer to the transfiguration of Jesus which is about to take place (Mark 9:2–8). The transfiguration is then to be regarded as the enthronement of Jesus that anticipates the glory that is to come. It was a momentary manifestation of the sovereign power of Jesus that pointed to the end-time when he will return in power. As such, the transfiguration of Jesus offered dramatic proof to the three disciples who witnessed it that the kingdom of God had come in the preaching and ministry of Jesus.2 This interpretation however, assumes that Jesus is only speaking to his disciples when he says these words and that the crowd has left.
The kingdom coming with power could refer to the establishment of the church as a spiritual kingdom.
1 And he said to them, “Truly, I say to you, there are some standing here who will not taste death until they see the kingdom of God after it has come with power.”