Part of his Slav Epic

http://www.muchafoundation.org/en/gallery/themes/theme/slav-epic/object/221

It's a huge canvas (~6m x 4m), and makes a massive impact when you see it.

From that foundation page:

"The Slav Epic' cycle No.10: The Meeting at Křížky. The Magic of Words: Sub utraque (1419) (1916) Following his death at the stake for his teachings, Jan Hus became a symbol of the Czech fight against the immoral conduct of the Catholic Church. Increasing numbers of Czech clergymen began to turn their back on papal rule and to deliver their sermons in the Czech language. They were declared heretics by the Papacy and the Council of Constance ordered that they be removed from their parishes. Charles University in Prague was also closed to ensure that their teaching ceased. Riots ensued and Hus’ followers began to gather in remote places outside the city walls in order to mount their rebellion.

Mucha depicts the most important of these gatherings which took place at Křížky, south of Prague, on 30 September 1419. Koranda, a radical preacher, called on Hus’ followers to take up their arms to defend their faith. He stands praying on a makeshift pulpit facing the throngs of followers as they arrive at Křížky. The dark sky above announces the imminent devastation of the Hussite Wars."