Head to the area in front of the small courtyard. Talk to the female Police Officer nearby. Talk with her to start the mission or to continue it forward by getting the law on your side.
Talk with the Police Officer you can tell her about the Kidnapping and Coercion and the Forgery and Extortion. She will be tired of Drahomir getting away with what he has been doing. She will go and charge the location with a few nearby officers.
If you want to deal with Drahomir yourself, a Sniper Rifle is recommended. This will convince most of the cops in Prague that you are going to hunting them. Just snipe him from as far away as possible. After that, make a run for the sewers or your apartment. This is very rewarding, but also very challenging.