On reaching Jerusalem, Jesus entered the temple courts and began driving out those who were buying and selling there.
He overturned the tables of the money changers and the benches of those selling doves*, and would not allow anyone to carry merchandise through the temple courts. And as he taught them, he said, “Is it not written: ‘My house will be called a house of prayer for all nations’? But you have made it ‘a den of robbers.’”
The chief priests and the teachers of the law heard this and began looking for a way to kill him, for they feared him, because the whole crowd was amazed at his teaching.
*Doves - the offering specifically mentioned by Jesus - were the offering required to be paid by the poor folk, who'd often bring in their own doves, only to find out they were not "pure" enough for a sacrifice and thus, the poor were forced to pay more - that they couldn't afford - in order to be in God's Temple. Thus, the "den of robbers" charge, especially insofar as they were ripping off especially the poor, who could least afford it.
It is this action that was at least one of the final straws for the religious authorities that drove them to plan Jesus' execution by the state.