Air carrier Captain reported unintentional aircraft movement at the gate when the Captain unknowingly released the parking brake; and the aircraft started moving. The ramp agent notified the Captain and the brakes were applied with no injury or damage.

Narrative

We were in contact with the mechanic on the intercom; doors closed; chocks in; and brakes set. Checklists have not been run yet; and having just finished the briefing; I re-adjusted my pedals and was about to call for the Preflight Checklist. In the re-adjustment of the pedals; I shifted in my seat; and without realizing it; accidentally touched the brakes; and they released. A few seconds later; the mechanic on the intercom said 'set the brakes'. I replied 'brakes are set'; because they were confirmed set with him a few minutes prior; and as I was speaking; I turned my gaze and saw they were released. I quickly set the parking brake; but the brake pressure was low and we were in motion. I immediately turned on an Aux pump and we came to a stop. Nothing was in the way; no one was injured; and no damage occurred. Where were the chocks?Cause: Mishaps could happen; but we have guards in place for that. Those guards would be the chocks to be in place. The ground crew removed the chocks without asking and without the hydraulic system pressurized; so the brake pressure was insufficient even-though the parking brake was set initially. If the chocks would have been in place; we would not have moved.Suggestions: Chocks are supposed to be in place to guard against any mishaps or movement. Going forward; I will get verbal confirmation that chocks are not removed without my consent. A safety procedure should be put in place requiring the following:Ground crew cannot remove chocks until the hydraulic system is pressurized to hold the brakes; and the mechanic on the intercom acknowledges with the pilot that it's safe to remove the chocks. This will remove any chance of a mishap and greatly increase safety.