He said he was protesting "police repression and state violence."
He filmed his climb, which led to the cancellation of tours of Parliament and the closure of the Westminster Bridge.
About three hours after the man reached a ledge on the tower, three emergency personnel were lifted up on an aerial ladder platform to speak with him.
The negotiations continued with no sign of the man agreeing to come down 10 hours later, when Big Ben chimed after nightfall at 21:00 GMT, or 4 p.m. ET.
The man's climb began when he entered the grounds of Parliament and walked up some stairs. He then climbed onto a railing and started scaling Elizabeth Tower.
The climber took his shoes off at one point so he could go higher on the tower, and authorities said he injured his feet, with pools of blood being seen on the tower's stonework.