Protesters marched on an Interior Ministry military base in Maruipol overnight, confronting the troops there and calling on them to abandon the site, which is along Donetsk Province’s southern coast.
The troops opened fire on the protesters, hitting several and starting a riot, as protesters began hurling petrol bombs at the base, causing yet more fire. In the end, at least three protesters were killed and 13 wounded, some of them severely.
The military also reportedly captured over 60 of the protesters at the site, though they later released many of them, conceding there was nothing they could actually charge them with.
Whether this is the single deadliest incident of the Ukrainian invasion of the east is disputed, as some reports of the offensive against Kramatorsk airfield said four people were dead, though since then there have been some doubts as to whether those reports were accurate.
Note the difference with Donetsk. Clearly no mercenaries in Mariupol (yet!). Just a local riot organised by the pro-Russian minority, hoping to provoke Putin's intervention on their behalf. Mariupol is 70% ethnic Ukrainian and 50% Russian speaking. It also just happens to be one of the six provinces that Putin would have to grab to get control of the Kharkov – Sebastopol railway line.