Sunday's match at the San Siro was suspended twice and the referee ordered the players off the field during the second half.
Play was halted in the first half for a few minutes and then late in the match it was stopped for 10 minutes, when there also appeared to be a small fire in the stands. Riot police waded into the stands and eventually the final 17 minutes were played.
The match finished 1-1.
Croatia coach Niko Kovac, who pleaded with fans to calm down, says, "this is not football. This is not a fair image of our country or our people."