Some were held for a day without being told why. All the police had to do is walk up to them and ask for ID. By law if you are walking in a public place you don't need to produce identification, but for some strange reason the week of the G20 meeting in downtown Toronto you did. If not, off you went to a holding area with no further explanation.
There are at least two class action lawsuits underway against the police and various levels of government over the treatment.