Detective Constable MarkAlstonis accused of using unreasonable force against Alfie Meadows, then 20, who suffered a brain injury requiring emergency surgery.
Video footage is said to capture the moment Dc MarkAlstonraised his weapon in a “violent, uncontrolled and dangerous manner” before striking Mr Meadows.
Then studying philosophy at Middlesex University, he was among more than 10,000 people demonstrating against student fees in London on December 9 2010.
DcAlston, then of Pc rank, was deployed to police the protest, which turned violent in and around Parliament Square.
The hearing was told senior officers were concerned about the safety of their colleagues and feared of a threat to their lives.
In statements, Mr Meadows described seeing an officer “thrashing about” before feeling a sharp strike to his head after he turned his back.
He needed more than 100 staples in his head and was left with a large scar but was charged with causing violent disorder and taken to court where he was cleared by a jury in 2013.
A probe by the police watchdog, now called the Independent Office for Police Conduct (IOPC), found DcAlstonhad a case to answer for gross misconduct but City of London police rejected the recommendation.